fix(terminal): rewrite A && B & to A && { B & } to prevent subshell leak

bash parses `A && B &` with `&&` tighter than `&`, so it forks a subshell
for the compound and backgrounds the subshell. Inside the subshell, B
runs foreground, so the subshell waits for B. When B is a process that
doesn't naturally exit (`python3 -m http.server`, `yes > /dev/null`, a
long-running daemon), the subshell is stuck in `wait4` forever and leaks
as an orphan reparented to init.

Observed in production: agents running `cd X && python3 -m http.server
8000 &>/dev/null & sleep 1 && curl ...` as a "start a local server, then
verify it" one-liner. Outer bash exits cleanly; the subshell never does.
Across ~3 days of use, 8 unique stuck-terminal events and 7 leaked
bash+server pairs accumulated on the fleet, with some sessions appearing
hung from the user's perspective because the subshell's open stdout pipe
kept the terminal tool's drain thread blocked.

This is distinct from the `set +m` fix in 933fbd8f (which addressed
interactive-shell job-control waiting at exit). `set +m` doesn't help
here because `bash -c` is non-interactive and job control is already
off; the problem is the subshell's own internal wait for its foreground
B, not the outer shell's job-tracking.

The fix: walk the command shell-aware (respecting quotes, parens, brace
groups, `&>`/`>&` redirects), find `A && B &` / `A || B &` at depth 0
and rewrite the tail to `A && { B & }`. Brace groups don't fork a
subshell — they run in the current shell. `B &` inside the group is a
simple background (no subshell wait). The outer `&` is absorbed into
the group, so the compound no longer needs an explicit subshell.

`&&` error-propagation is preserved exactly: if A fails, `&&`
short-circuits and B never runs.

- Skips quoted strings, comment lines, and `(…)` subshells
- Handles `&>/dev/null`, `2>&1`, `>&2` without mistaking them for `&`
- Resets chain state at `;`, `|`, and newlines
- Tracks brace depth so already-rewritten output is idempotent
- Walks using the existing `_read_shell_token` tokenizer, matching the
  pattern of `_rewrite_real_sudo_invocations`

Called once from `BaseEnvironment.execute` right after
`_prepare_command`, so it runs for every backend (local, ssh, docker,
modal, etc.) with no per-backend plumbing.

34 new tests covering rewrite cases, preservation cases, redirect
edge-cases, quoting/parens/backticks, idempotency, and empty/edge
inputs. End-to-end verified on a test VM: the exact vela-incident
command now returns in ~1.3s with no leaked bash, only the intentional
backgrounded server.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tools/terminal_tool.py

@@ -442,6 +442,171 @@ def _rewrite_real_sudo_invocations(command: str) -> tuple[str, bool]:
     return "".join(out), found
 
 
+def _rewrite_compound_background(command: str) -> str:
+    """Wrap `A && B &` (or `A || B &`) to `A && { B & }` at depth 0.
+
+    Bash parses ``A && B &`` with `&&` tighter than `&`, so it forks a
+    subshell for the whole `A && B` compound and backgrounds it. Inside
+    the subshell, `B` runs foreground, so the subshell waits for `B` to
+    finish. When `B` is a long-running process (`python3 -m http.server`,
+    `yes > /dev/null`, anything that doesn't naturally exit), the subshell
+    never exits. It leaks as a process stuck in ``wait4`` forever — and
+    on the way, its open stdout pipe can prevent the terminal tool from
+    returning promptly.
+
+    Rewriting the tail to `A && { B & }` preserves `&&`'s error semantics
+    (skip B if A fails) while replacing the subshell with a brace group.
+    The brace group runs in the current shell (no fork), backgrounds B as
+    a simple command (bash doesn't wait for it in non-interactive mode),
+    and exits immediately. B runs as a normal backgrounded child, orphaned
+    when the parent shell exits.
+
+    Handles redirects (``&>``, ``2>&1``) and skips content inside quoted
+    strings and parenthesised subshells. Leaves simple ``cmd &`` alone —
+    that construct doesn't have the subshell-wait bug.
+    """
+    n = len(command)
+    i = 0
+    paren_depth = 0
+    brace_depth = 0
+    # Position in *command* just after the most recent `&&` / `||` at depth 0
+    # in the current statement; -1 when no chain operator is active.
+    last_chain_op_end = -1
+    rewrites: list[tuple[int, int]] = []  # (chain_op_end, amp_pos)
+
+    while i < n:
+        ch = command[i]
+
+        # Newline terminates a statement at depth 0 — reset chain state.
+        # Checked before the whitespace skip so we don't miss it.
+        if ch == "\n" and paren_depth == 0 and brace_depth == 0:
+            last_chain_op_end = -1
+            i += 1
+            continue
+
+        if ch.isspace():
+            i += 1
+            continue
+
+        # Comments (only at statement start — conservative: any `#` not inside
+        # a token ends the line). `_read_shell_token` handles quoted strings
+        # below so `#` inside quotes is safe.
+        if ch == "#":
+            nl = command.find("\n", i)
+            if nl == -1:
+                break
+            i = nl
+            continue
+
+        if ch == "\\" and i + 1 < n:
+            i += 2
+            continue
+
+        # Quoted tokens — consume whole string via the shared tokenizer.
+        if ch in ("'", '"'):
+            _, next_i = _read_shell_token(command, i)
+            i = max(next_i, i + 1)
+            continue
+
+        if ch == "(":
+            paren_depth += 1
+            i += 1
+            continue
+
+        if ch == ")":
+            paren_depth = max(0, paren_depth - 1)
+            i += 1
+            continue
+
+        # Brace groups: `{ ... }` is a group (no subshell fork), and bash
+        # requires whitespace after `{`. We track depth so already-rewritten
+        # output (`A && { B & }`) is idempotent — the inner `&` is part of
+        # the group, not a new compound to rewrite. Also skip content inside
+        # the group since `A && B &` there is separately well-formed.
+        if ch == "{" and i + 1 < n and (command[i + 1].isspace() or command[i + 1] == "\n"):
+            brace_depth += 1
+            i += 1
+            continue
+        if ch == "}" and brace_depth > 0:
+            brace_depth -= 1
+            # Closing a group completes a compound statement; reset chain.
+            last_chain_op_end = -1
+            i += 1
+            continue
+
+        # Inside parens or brace groups, skip operators — they parse in their
+        # own scope. `(...)` subshells have the same bug class but are not the
+        # common agent pattern; leave for a follow-up.
+        if paren_depth > 0 or brace_depth > 0:
+            i += 1
+            continue
+
+        # Chain operators at depth 0
+        if command.startswith("&&", i) or command.startswith("||", i):
+            last_chain_op_end = i + 2
+            i += 2
+            continue
+
+        # Statement terminators reset the chain state
+        if ch == ";":
+            last_chain_op_end = -1
+            i += 1
+            continue
+
+        # Single `|` (pipe) starts a new pipeline stage; don't rewrite
+        # across it. `||` handled above.
+        if ch == "|":
+            last_chain_op_end = -1
+            i += 1
+            continue
+
+        # `&` handling: distinguish `&&`, `&>`, fd redirect (`>&`, `<&`),
+        # and a true backgrounding `&`.
+        if ch == "&":
+            # `&&` handled above; won't reach here
+            if i + 1 < n and command[i + 1] == ">":
+                # `&>` redirect — consume
+                i += 2
+                continue
+            # `>&` / `<&` fd target — look back past whitespace
+            j = i - 1
+            while j >= 0 and command[j].isspace():
+                j -= 1
+            if j >= 0 and command[j] in "<>":
+                i += 1
+                continue
+            # Real background operator
+            if last_chain_op_end >= 0:
+                rewrites.append((last_chain_op_end, i))
+            last_chain_op_end = -1
+            i += 1
+            continue
+
+        # Regular unquoted token — advance past it via the shared tokenizer
+        _, next_i = _read_shell_token(command, i)
+        i = max(next_i, i + 1)
+
+    if not rewrites:
+        return command
+
+    # Apply rewrites back-to-front so earlier indices remain valid.
+    result = command
+    for chain_end, amp_pos in reversed(rewrites):
+        # Skip whitespace right after the `&&`/`||` so the brace group
+        # opens flush against the inner command.
+        insert_pos = chain_end
+        while insert_pos < amp_pos and result[insert_pos].isspace():
+            insert_pos += 1
+        prefix = result[:insert_pos]
+        middle = result[insert_pos:amp_pos]  # inner command + trailing space
+        suffix = result[amp_pos + 1 :]
+        # `{` needs a trailing space in bash; the closing `}` needs to be
+        # preceded by `;` or `&` — we're providing `&` from the backgrounding.
+        result = prefix + "{ " + middle + "& }" + suffix
+
+    return result
+
+
 def _transform_sudo_command(command: str | None) -> tuple[str | None, str | None]:
     """
     Transform sudo commands to use -S flag if SUDO_PASSWORD is available.