Add experimental Activity page

[nwparker]

Summary

• add an experimental Activity page behind the Activity Page setting
• group agent/worktree status events into Slack-style worktree threads with unread controls
• preserve worktree agent status visibility and add tests for retained/explicit agent rows

Performance review

• Activity is default-off and hidden unless the experimental setting is enabled
• Activity page is lazy-mounted and only renders when the view is active
• sidebar unread count returns immediately while disabled and avoids flattened worktree allocations when enabled
• worktree/repo maps use existing WeakMap selector caches

Verification

• pnpm run tc:web
• pnpm run tc:node
• pnpm run lint (existing GitHub Project warnings only)
• pnpm exec vitest run src/renderer/src/lib/worktree-status.test.ts --config config/vitest.config.ts