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.

hermes_cli/commands.py

@@ -211,8 +211,8 @@ COMMAND_REGISTRY: list[CommandDef] = [
     CommandDef("debug", "Upload debug report (system info + logs) and get shareable links", "Info"),
 
     # Exit
-    CommandDef("quit", "Exit the CLI", "Exit",
-               cli_only=True, aliases=("exit",)),
+    CommandDef("quit", "Exit the CLI (use --delete to also remove session history)", "Exit",
+               cli_only=True, aliases=("exit",), args_hint="[--delete]"),
 ]