feat(ai): List saved assistant sessions so past chats can be resumed

[nfebe]

Assistant conversations were already persisted per user, but nothing could enumerate them, so the UI could not show past chats or reopen one. This adds a listing of the caller's saved sessions (all of them for an admin), most recent first, each with a short title from its first message. It reuses the existing on-disk session store.

UI counterpart: flatrun/ui#88.

[sourceant]

Code Review Summary

This PR implements a feature to list saved AI sessions. It introduces a lightweight summary view, smart title generation from chat history, and an API handler that correctly filters sessions based on user identity and admin status.

🚀 Key Improvements

• Added SessionSummary to avoid transferring full transcripts in list views.
• Implemented intelligent title generation from the first visible user message.
• Added unit tests for title generation and store listing logic.
• Properly integrated admin/user visibility filtering in the API layer.

💡 Minor Suggestions

• Consider passing a filter (e.g., UserID) to SessionStore.List to avoid loading all sessions from disk just to filter them in memory at the API handler level.
• Use slices.SortFunc if the project uses Go 1.21+.

[sourceant]

Review complete. See the overview comment for a summary.

[sourceant]

While functional, this implementation has a few performance and efficiency considerations:

1. Performance: st.Get unmarshals the full session JSON, including potentially large transcripts (hundreds of messages). For a listing view, consider decoding into a 'metadata-only' struct that ignores the messages array to save memory and CPU.
2. Optimization: Pre-allocate the summaries slice capacity to avoid repeated re-allocations during the loop.
3. Modern Go: If using Go 1.21+, slices.SortFunc is preferred over sort.Slice for type safety and performance.

Suggested change

	summaries := []SessionSummary{}
	summaries := make([]SessionSummary, 0, len(entries))
	for _, e := range entries {
	if e.IsDir() || !strings.HasSuffix(e.Name(), ".json") {
	continue
	}
	// Note: st.Get loads the full session. Optimization to load metadata only is recommended for scale.
	sess, err := st.Get(strings.TrimSuffix(e.Name(), ".json"))
	if err != nil {
	continue
	}
	summaries = append(summaries, sess.Summary())
	}
	sort.Slice(summaries, func(i, j int) bool { return summaries[i].UpdatedAt.After(summaries[j].UpdatedAt) })