feat(cli): add /exit --delete flag to remove session on quit (#27101)

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.

cli.py

@@ -2824,6 +2824,11 @@ class HermesCLI:
         # turn (which would make Ctrl+C feel like it did nothing).
         self._last_turn_interrupted = False
         self._should_exit = False
+        # /exit --delete: when True, the current session's SQLite history and
+        # on-disk transcripts are deleted during shutdown. Set by
+        # process_command() when the user runs /exit --delete or /quit --delete.
+        # Ported from google-gemini/gemini-cli#19332.
+        self._delete_session_on_exit = False
         self._last_ctrl_c_time = 0
         self._clarify_state = None
         self._clarify_freetext = False
@@ -7653,6 +7658,16 @@ class HermesCLI:
         canonical = _cmd_def.name if _cmd_def else _base_word
         
         if canonical in {"quit", "exit"}:
+            # Parse --delete flag: /exit --delete also removes the current
+            # session's transcripts + SQLite history. Ported from
+            # google-gemini/gemini-cli#19332.
+            _rest = cmd_original.split(None, 1)
+            _args = (_rest[1] if len(_rest) > 1 else "").strip().lower()
+            if _args in ("--delete", "-d"):
+                self._delete_session_on_exit = True
+            elif _args:
+                _cprint(f"  {_DIM}✗ Unknown argument: {_escape(_args)}. Use /exit --delete to also remove session history.{_RST}")
+                return True
             return False
         elif canonical == "help":
             self.show_help()
@@ -13822,6 +13837,19 @@ class HermesCLI:
                     self._session_db.end_session(self.agent.session_id, "cli_close")
                 except (Exception, KeyboardInterrupt) as e:
                     logger.debug("Could not close session in DB: %s", e)
+                # /exit --delete: also remove the current session's transcripts
+                # and SQLite history. Ported from google-gemini/gemini-cli#19332.
+                if getattr(self, '_delete_session_on_exit', False):
+                    try:
+                        from hermes_constants import get_hermes_home as _ghh
+                        _sessions_dir = _ghh() / "sessions"
+                        _sid = self.agent.session_id
+                        if self._session_db.delete_session(_sid, sessions_dir=_sessions_dir):
+                            _cprint(f"  {_DIM}✓ Session {_escape(_sid)} deleted{_RST}")
+                        else:
+                            _cprint(f"  {_DIM}✗ Session {_escape(_sid)} not found for deletion{_RST}")
+                    except (Exception, KeyboardInterrupt) as e:
+                        logger.debug("Could not delete session on exit: %s", e)
             # Plugin hook: on_session_end — safety net for interrupted exits.
             # run_conversation() already fires this per-turn on normal completion,
             # so only fire here if the agent was mid-turn (_agent_running) when