feat(desktop): add per-thread agent activity map

[zrmarley]

Summary

Adds an experimental, per-thread activity map for supervising agent file work in Buzz.

• Places an Activity pill beside a thread's reply summary when one or more managed agents performed observable file work in that thread.
• Opens a map scoped to the selected thread, merging each participating agent's reads and writes into one gap-compressed timeline.
• Supports live following and DVR-style replay: play/pause, two-second scrubbing, playback speed, restart, and keyboard controls.
• Shows per-agent riders and filters, with click-through to the corresponding agent session.
• Highlights nearby writes to the same file by multiple agents as contention.
• Hydrates retained observer events from the local archive and resolves each turn to its triggering thread without introducing a new relay event kind.
• Keeps the feature behind the existing agentActivityPanel experimental flag.

The thread pill is intentionally the scope selector: there is no channel-wide aggregate or thread picker inside the popover, which keeps activity attached to the conversation that initiated it.

Screenshots

Per-thread entry points


Idle multi-agent map, including contention on store.rs


Live activity at the leading edge


All names, messages, paths, and activity data shown above are synthetic.

Related issue

N/A — discussed and prototyped internally before implementation.

Testing

• pnpm check
• pnpm test — 6,561 tests passed after rebasing onto current main
• pnpm build:e2e
• pnpm exec playwright test tests/e2e/agent-activity-panel-screenshots.spec.ts --project=smoke — 5 tests passed
• Pre-push hooks passed: file-size check, desktop check, desktop typecheck, and desktop tests
• Manually exercised with multiple live agents in one channel

Deferred follow-ups

• Add an accessible text representation of canvas file/event data.
• Add an explicit keyboard-operable session-navigation affordance beyond rider clicks and chip double-click.
• Consider bounded same-mount retry/backoff for transient thread-event lookup failures; rejected lookups already evict so later mounts can retry.
• Stabilize a filtered agent's color index across filtered and unfiltered scenes.

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

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