fix: preserve session_db on UNIQUE conflict and add /resume CLI command

[Mibayy]

Fixes #3123 — two separate bugs in the --resume / /resume flow.

Bug 1: sessions not persisted (messages lost on --resume)

Root cause: The CLI creates the SQLite session row early in __init__. Then AIAgent.__init__ calls create_session() again for the same session ID, hits a UNIQUE constraint, and the exception handler sets self._session_db = None. From that point every append_message() call short-circuits — messages accumulate in memory only and are never written to the DB. On --resume, the session row exists but is empty ("found but has no messages").

Fix (run_agent.py): Distinguish UNIQUE/already-exists errors (expected when the caller pre-creates the session) from genuine failures. Only discard session_db for unexpected errors; silently skip for UNIQUE conflicts.

Bug 2: /resume unknown command in CLI

Root cause: _handle_resume_command exists only in gateway/run.py. The CLI slash-command dispatcher had no handler for resume, even though it's listed in commands.py without gateway_only=True.

Fix (cli.py): Add _handle_resume_command() to HermesCLI and wire it to canonical == "resume" in the dispatcher.

The new CLI handler supports:

• /resume — lists recent named sessions
• /resume <session-id> — switches to a session by its timestamp ID
• /resume <title> — resolves by title (via resolve_session_by_title)

Flushes memories and ends the current session in the DB before switching, mirroring the gateway behaviour.

[teknium1]

Thanks for this diagnosis and the clear write-up, @Mibayy! Both bugs you identified have since been fixed on main via separate commits.

Bug 1 — session_db nulled on UNIQUE conflict (run_agent.py):
The current main takes an even broader approach: the create_session exception handler no longer nulls self._session_db at all. It logs a warning and keeps the DB reference alive in all error cases (run_agent.py, lines 1568–1577). This fully covers the UNIQUE-conflict scenario you diagnosed.

Bug 2 — /resume unknown command in CLI (cli.py):
_handle_resume_command() already exists at cli.py:4732 and is wired into the dispatcher at cli.py:6035. The current implementation is richer than this PR's version — it handles compression-chain following via resolve_resume_session_id, title lookup via _resolve_session_by_name_or_id, and reopen_session() to clear the ended_at timestamp. Prior commits f24956ba1, bd01ec788, and a456b50a3 document the history.

Closing as implemented on main. This is an automated hermes-sweeper review.