fix(ui): event count + relative time in lane; evict stale tail states

[integrate-your-mind]

Summary

• AgentListItem: active agents show current "doing" text; idle agents fall back to N events · Xm ago so the lane is never blank
• formatRelativeTime: new helper — just now / Xm / Xh / Xd ago
• AgentLane: shows 3 animated skeleton items while WebSocket is connecting instead of an empty list
• evictStaleTails: removes TailState map entries for session paths no longer seen in the scan — prevents unbounded memory growth; matches the pattern already used for activityCache / pidSessionCache / opencodeSessionByPidCache
• scan.ts: calls evictStaleTails(tailTargets) at end of each full scan

What was already correct (no changes needed)

• Page refresh: full WS snapshot is pushed immediately on reconnect
• Duplicate sessions: dedupeAgents + pickBetterAgent already handles this
• Race conditions in session assignment: reuseBlocked + markOpenCodeSessionUsed guards already in place

Test plan

• Build passes: tsc --noEmit
• Unit tests: 74 tests, 71 pass, 0 fail
• Idle agents in the lane now show N events · Xm ago instead of blank meta
• Connecting state shows 3 skeleton items with shimmer animation
• evictStaleTails is called after each scan; old sessions freed from memory

🤖 Generated with Claude Code

[chatgpt-codex-connector]

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[integrate-your-mind]

Backlog triage: this PR is a valid stacked follow-up to #6, not a duplicate. Commit comparison shows it is one commit ahead of 92321259cb17fd8dc574092339087959db5c6184 and its unique delta is limited to AgentLane/AgentListItem UI feedback, relative-time formatting, connecting skeletons, and stale Codex tail eviction. Retargeting the base to fix/activity-state so this PR displays only its unique changes. After #6 merges, this PR should be rebased or retargeted to main.

[integrate-your-mind]

Returning this stacked PR to draft while #6 is blocked. Its base is fix/activity-state, so it cannot become the next merge target until #6's Codex expiry bug is fixed, tested, and merged. After that, retarget this PR to main; its review surface should remain one commit and seven files.