Add collapsible active-sessions sidebar to the session page

[ai-yappa]

Summary

Adds a collapsible sidebar to the Codevil session page (/session/:id) that shows all other active sessions — sessions currently running or awaiting review — with the current session excluded.

What changed

• packages/web/src/components/session/session-sidebar.tsx (new)

  • Fetches the session directory via the existing listSessions endpoint on mount.
  • Filters to active sessions (running / awaiting review) and excludes the currently open session, sorted most-recent first.
  • Collapsible: a chevron toggle collapses the panel to a narrow ribbon that keeps a live active-count badge. The collapsed state persists in localStorage (codevil_session_sidebar_collapsed).
  • Refreshes every 30s and whenever the tab regains focus/visibility, so the "other active sessions" list stays current across multiple sessions.
  • Each entry shows title, status pill, repo, and relative last-activity time, and navigates to that session.
  • Empty state ("No other active sessions") when nothing else is active.

• packages/web/src/lib/session-status.ts (new)

  • Extracts the session-status derivation (deriveStatus, STATUS_LABEL) that lived in the home page so the sidebar and home page share one status vocabulary.
  • Adds isActiveSession (running or awaiting review; archived/failed/done are excluded) and filterOtherActiveSessions.

• packages/web/src/routes/index.tsx — refactored to import the shared status module (no behavior change).

• packages/web/src/routes/session.$id.tsx — renders the sidebar between the top rail and the workbench via a new .session-body row.

• packages/web/src/session-components.css — sidebar styles (expanded/collapsed widths, dark-mode friendly via existing theme vars, responsive sizing ≤900px).

• packages/web/src/lib/__tests__/session-status.test.ts (new) — unit tests for status derivation, active filtering, and "other active sessions" selection.

Verification

• pnpm typecheck (repo root): passes for all packages.
• pnpm test in packages/web: 264 tests pass (25 files), including the 10 new ones.
• Root pnpm verify runs the sandbox-image integration tests, which fail in this sandbox (14 failures in packages/sandbox-image, environment-dependent: clone/pr/agent-tool behaviors). These are unrelated to this web-only change; web typecheck + build + tests all pass.