feat(cli): add /exit --delete flag to remove session on quit

Port from google-gemini/gemini-cli#19332.

Users can now exit with '/exit --delete' (or '/quit --delete', '/exit -d')
to permanently remove the current session's SQLite history plus on-disk
transcripts (*.json / *.jsonl / request_dump_*) in one shot. Useful for
privacy-sensitive workflows and one-off interactions where leaving a
session recording behind is undesirable.

Implementation:
- New HermesCLI._delete_session_on_exit one-shot flag (defaults False).
- process_command() parses --delete / -d after /exit or /quit and arms
  the flag. Unknown args print a hint and keep the CLI running (prevents
  typos like '/exit -delete' from accidentally exiting).
- Shutdown path calls SessionDB.delete_session(session_id, sessions_dir=...)
  right after end_session() when the flag is set. That API already
  existed for 'hermes sessions delete' and handles both SQLite removal
  (orphaning child sessions so FK constraints hold) and on-disk file
  cleanup.
- /quit CommandDef now advertises '[--delete]' in args_hint so /help
  and CLI autocomplete surface it.

Tests: tests/cli/test_exit_delete_session.py (12 cases covering both
aliases, case insensitivity, whitespace, short form, unknown-arg
rejection, and registry metadata).

E2E-verified with isolated HERMES_HOME: session row deleted, all three
transcript/request-dump files removed, second delete_session call
correctly returns False.

tests/cli/test_exit_delete_session.py

@@ -0,0 +1,119 @@
+"""Tests for `/exit --delete` and `/quit --delete` session deletion.
+
+Ports the behavior from google-gemini/gemini-cli#19332: running `/exit` or
+`/quit` with the `--delete` flag arms a one-shot `_delete_session_on_exit`
+flag that the CLI shutdown path uses to remove the current session from
+SQLite + on-disk transcripts before exit.
+"""
+
+from unittest.mock import MagicMock
+
+
+def _make_cli():
+    """Bare HermesCLI suitable for process_command() tests.
+
+    Uses ``__new__`` to skip the heavy __init__; only sets the attributes
+    the /exit branch touches.
+    """
+    from cli import HermesCLI
+    cli = HermesCLI.__new__(HermesCLI)
+    cli.config = {}
+    cli.console = MagicMock()
+    cli.agent = None
+    cli.conversation_history = []
+    cli.session_id = "test-session"
+    cli._delete_session_on_exit = False
+    return cli
+
+
+class TestExitDeleteFlag:
+    def test_plain_exit_does_not_arm_delete(self):
+        cli = _make_cli()
+        result = cli.process_command("/exit")
+        assert result is False
+        assert cli._delete_session_on_exit is False
+
+    def test_plain_quit_does_not_arm_delete(self):
+        cli = _make_cli()
+        result = cli.process_command("/quit")
+        assert result is False
+        assert cli._delete_session_on_exit is False
+
+    def test_exit_delete_arms_flag(self):
+        cli = _make_cli()
+        result = cli.process_command("/exit --delete")
+        assert result is False
+        assert cli._delete_session_on_exit is True
+
+    def test_quit_delete_arms_flag(self):
+        cli = _make_cli()
+        result = cli.process_command("/quit --delete")
+        assert result is False
+        assert cli._delete_session_on_exit is True
+
+    def test_exit_delete_short_form(self):
+        """`-d` is a convenience alias for `--delete`."""
+        cli = _make_cli()
+        result = cli.process_command("/exit -d")
+        assert result is False
+        assert cli._delete_session_on_exit is True
+
+    def test_quit_alias_q_is_not_quit(self):
+        """`/q` is the alias for `/queue`, not `/quit`. This test documents
+        that /q --delete does NOT arm session deletion — it would dispatch
+        to /queue instead."""
+        cli = _make_cli()
+        cli._pending_input = __import__("queue").Queue()
+        # /q with no args shows a usage error and keeps the CLI running.
+        result = cli.process_command("/q")
+        assert result is not False  # queue command doesn't exit
+        assert cli._delete_session_on_exit is False
+
+    def test_delete_flag_is_case_insensitive(self):
+        cli = _make_cli()
+        result = cli.process_command("/exit --DELETE")
+        assert result is False
+        assert cli._delete_session_on_exit is True
+
+    def test_delete_flag_trims_whitespace(self):
+        cli = _make_cli()
+        result = cli.process_command("/exit   --delete   ")
+        assert result is False
+        assert cli._delete_session_on_exit is True
+
+    def test_unknown_exit_argument_does_not_exit(self):
+        """Unrecognised args should NOT exit the CLI — they surface an
+        error message and stay in the session. This prevents accidental
+        session destruction from typos like `/exit -delete`."""
+        cli = _make_cli()
+        result = cli.process_command("/exit --delte")
+        # process_command returns True = keep running
+        assert result is True
+        assert cli._delete_session_on_exit is False
+
+    def test_unknown_exit_argument_prints_help(self):
+        cli = _make_cli()
+        # _cprint goes through module-level print, so capture via console.
+        # We can't patch _cprint directly without import juggling; the
+        # previous assertion already proves the unknown-arg branch is
+        # reached (result True + flag False).
+        result = cli.process_command("/exit garbage")
+        assert result is True
+        assert cli._delete_session_on_exit is False
+
+
+class TestCommandRegistry:
+    def test_quit_command_advertises_delete_flag(self):
+        """The CommandDef args_hint should surface `--delete` in /help and
+        CLI autocomplete."""
+        from hermes_cli.commands import resolve_command
+        cmd = resolve_command("quit")
+        assert cmd is not None
+        assert cmd.args_hint == "[--delete]"
+
+    def test_exit_alias_resolves_to_quit_with_hint(self):
+        from hermes_cli.commands import resolve_command
+        cmd = resolve_command("exit")
+        assert cmd is not None
+        assert cmd.name == "quit"
+        assert cmd.args_hint == "[--delete]"