fix(terminal): suppress declare -x stdout leak when sourcing snapshot on macOS

On macOS (bash 3.2 and certain Homebrew builds), sourcing a file
containing declare -x statements prints each declaration to stdout.
The session snapshot (written by export -p) is full of these, so
every command execution prepends ~60 lines of env vars into the LLM's
context window, wasting tokens and potentially exceeding context
limits (issue #15459).

Fix: redirect both stdout and stderr to /dev/null when sourcing the
snapshot. The source builtin still sets the variables in the current
shell — only the print output is suppressed. On Linux this is already
silent, so the redirect is harmless.

Fixes #15459

tools/environments/base.py

@@ -386,9 +386,14 @@ class BaseEnvironment(ABC):
 
         parts = []
 
-        # Source snapshot (env vars from previous commands)
+        # Source snapshot (env vars from previous commands).
+        # Redirect stdout to /dev/null because on macOS, sourcing a file
+        # that contains `declare -x` statements prints each declaration to
+        # stdout, leaking 60+ lines of env vars into the LLM context
+        # (issue #15459). On Linux this is silent, so the redirect is
+        # harmless there.
         if self._snapshot_ready:
-            parts.append(f"source {self._snapshot_path} 2>/dev/null || true")
+            parts.append(f"source {self._snapshot_path} > /dev/null 2>&1 || true")
 
         # Preserve bare ``~`` expansion, but rewrite ``~/...`` through
         # ``$HOME`` so suffixes with spaces remain a single shell word.