feat: show active Pi sessions in project rail

[klioen]

Summary

• show a compact loading indicator on a Worktree while one of its Pi Chat sessions is streaming
• aggregate Worktree activity onto the owning Project, including directory Projects with hidden synthetic workspaces
• keep Chat session attachments alive when switching tabs so background completion updates the indicator accurately
• clean up activity when sessions, Worktrees, or Projects are removed

Behavior

• active means agent_start through agent_settled
• idle, loading, failed, and auth-required sessions do not show activity
• multiple sessions aggregate to one boolean indicator; it disappears only after the last active session settles
• no polling, new IPC, or durable state was added

Verification

• pnpm check
• 63 Vitest files, 411 tests passed
• release tests passed
• macOS app and DMG smoke tests passed
• installed app signature and app.asar hash verified locally