feat(ui): optional last-update timestamp badge on session rows

[rodrimendoza]

Summary

Adds an opt-in dim Nm ago / Nh ago / Nd ago badge after each session row, showing the most recent of:

1. Instance.CreatedAt (SQLite, persisted floor)
2. Instance.LastStartedAt (SQLite, lifecycle restart)
3. Latest hook event UpdatedAt (StatusFileWatcher, present when agent-deck hooks are installed in Claude/Codex settings)
4. Confirmed tmux activity time (StateTracker.lastChangeTime, gated on a new realActivityConfirmed flag so freshly-built trackers don't falsely report "just now" for every session after restart)

The formula lives in pickBadgeTime (pure function) so it's unit-testable in isolation. LastAccessedAt is deliberately excluded — attaching to a quiet session to peek at it isn't an "update", and including it made the badge lie.

No pre-existing issue — happy to file one and discuss the design first if preferred.

Screenshots

Settings menu — toggle on (off by default)

Sessions list with the badge enabled

Why a new realActivityConfirmed flag in tmux.StateTracker?

Naively reading lastChangeTime is the obvious first attempt and was where I started. It doesn't work because tmux.go initializes lastChangeTime = time.Now() at tracker construction — so right after agent-deck starts, every session reports "just now" regardless of how long it's actually been idle.

The flag flips true only at the explicit-busy confirmation paths inside Session.GetStatus and Session.getStatusFallback (git grep 'realActivityConfirmed = true'), never at construction. Callers consult the bool before consuming the time value, so the "never observed" case falls back to the lifecycle floor.

How to enable

Settings → DISPLAY → Show session timestamps (default off). Persisted as [display] show_session_timestamps = true. Toggle takes effect immediately, no restart.

Risk

Render-path cost per row per tick: two in-memory accessor calls (hook watcher map + tmux confirmed activity) plus a handful of time.Time comparisons. No new I/O, no new goroutines. The StatusFileWatcher already runs for hook status display.

Test plan

• go build ./... clean
• New unit tests pin: the realActivityConfirmed flag (false at init, true after confirmation, zero-time sentinel when unobserved), the full pickBadgeTime formula across all 4 layers with nil/zero/unobserved edges, LastAccessedAt exclusion, nil tmuxSession safety, merge round-trip, settings-panel scroll mapping reaches the new row
• Manual verification: badge ticks live during an active Claude answer, freezes at last-active timestamp when Claude finishes, no false "just now" mass reset after agent-deck restart
• Maintainer manual verification

🤖 Generated with Claude Code

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