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.

website/docs/reference/slash-commands.md

@@ -87,7 +87,7 @@ Type `/` in the CLI to open the autocomplete menu. Built-in commands are case-in
 
 | Command | Description |
 |---------|-------------|
-| `/quit` | Exit the CLI (also: `/exit`). See note on `/q` under `/queue` above. |
+| `/quit` | Exit the CLI (also: `/exit`). See note on `/q` under `/queue` above. Pass `--delete` (or `-d`) — e.g. `/exit --delete` — to also permanently remove the current session's SQLite history and on-disk transcripts before exiting. Useful for privacy-sensitive or one-off tasks. |
 
 ### Dynamic CLI slash commands