fix(cli): ensure on_session_end hook fires on all exit paths

[kelsia14]

Problem

The on_session_end plugin hook was not firing on abrupt CLI exits, causing sessions to remain unindexed in FTS5. This affected three scenarios:

1. Ctrl+C (KeyboardInterrupt): Hook not called, session orphaned in JSON without SQLite index
2. SSH disconnect (BrokenPipeError): Not captured, process died without cleanup
3. SIGTERM/SIGHUP (kill/terminal hangup): No signal handlers, instant death

Solution

All exit paths now converge to the same finally block that invokes on_session_end.

Changes

• finally block: Added invoke_hook("on_session_end", ...) with interrupted=True
• except tuple: Added BrokenPipeError alongside KeyboardInterrupt
• Signal handlers: SIGHUP/SIGTERM now raise KeyboardInterrupt() to trigger cleanup

Flow

All exit paths (Ctrl+C, SSH drop, SIGTERM, SIGHUP, normal exit) now execute the same cleanup sequence including the on_session_end hook.

Testing

• pytest tests/hermes_cli/ -q: 755 passed
• pytest tests/test_plugins.py -q: 19 passed
• Manual: Verified hook fires on Ctrl+C with test plugin

Impact

Plugins can now reliably cleanup and sync session state on any exit type, fixing the "orphaned JSON sessions" issue.