feat(web): + menu replaces Mission Control; workbench view with activity, artifacts, approvals and docked composer (CL-8431)

[TheGreatAxios]

Summary

• Deletes Mission Control (page, route, sidebar row); /mission-control now redirects to /.
• The sidebar's top-right + is now a dropdown (react-ui Menu) with New Agent, New Chat, New Workbench — keeps data-tour="new-workbench-button" on the trigger.
• The workbench room (/w/:tenantId) gains a left info column: latest activity from the room thread, relevant artifacts (GET /api/tenants/:t/artifacts), pending approvals with approve/deny (folded in from Mission Control), and participants — the running chat stays in the main column with the composer docked at the bottom. Sub-thread panel behavior is unchanged.
• All data access goes through TanStack Query; the only useEffect is the existing SSE mailbox subscription.

Gaps / notes

• "New Workbench" opens the existing NEW_WORKBENCH_PATH (/new, NewWorkbenchPickerRoute) rather than a literal /w/new — /w/:id is already claimed by workbench room deep links, so a literal /w/new would collide with workbenchIdFromPath.
• Dropped Mission Control's "in flight runs" and "jump back in" panels (no native fires feed backs them, per the page's own dropped-KPI precedent); computeInFlightRows and its test were removed with the page. Only pending approvals were folded into the new view, per the ticket.
• No new tests added; updated pending-approvals.test.tsx and run-status-tone-parity.test.ts to drop their MissionControlRoute/computeInFlightRows references (kept their remaining coverage intact).

Test plan

• bun install
• bunx tsc -p apps/web --noEmit
• bun run fmt
• bun run lint
• bun run check (full repo: typecheck, lint, fmt, tests, web build) — green

[linear-code]

CL-8431