Add unit coverage for the #752 agents-view routing/hook layer

[selfcontained]

Summary

Recurring test-enforcer run. Local suite was green on first pass (10th consecutive run): check ok; unit server 2362 pass / 8 skip, web 481→500, extension 60, scripts 4; E2E 174 pass / 12 skip (run twice, 1.1m). CI scan clean — all recent failures are known branch-local issues, no flakes. With the suite stable, this run closes the next_focus coverage gap: the #752 whiteboard-era routing/hook layer, previously untested.

Tests added (19 → 18 after review dedup)

• apps/web/src/lib/agent-routes.test.ts — pins the three agent URL helpers to their literal paths, the contract shared by the router config in App.tsx, the useMatch patterns, and every navigate() call site.
• apps/web/src/hooks/use-agents-view-routing.test.tsx (11 tests) — drives the hook through a real MemoryRouter with a location/navigate probe instead of mocking the router:
  • invalid-agent redirect: waits for agentsLoaded, respects a validated id, no-ops on bare /agents, and its replace: true semantics are probed by clearing routeAgentId (mirroring useParams after the redirect) and asserting back-navigation cannot resurrect the dead agent URL
  • legacy /feedback/:itemId and /review/:summaryAgentId deep links collapse onto the agent route, gated on load
  • changesMatch/whiteboardMatch flags for all three tab locations
  • onTabChange navigation across all three tabs, its replace-not-push history behavior, and the no-route-agent no-op
• apps/web/src/hooks/use-whiteboard.test.tsx (6 tests) — query-key shape, enabled gate stays idle on a null agent id, versioned endpoint URL asserted, null→id transition starts the fetch, staleTime: Infinity remount serves cache without refetching, and per-agent cache separation.

Verification

• 13-mutant battery against the three source files: 13/13 caught. One mutant (dropping replace: true from the invalid-agent redirect) initially survived because the redirect effect re-fires and masks the pushed entry; the test was strengthened with a double back-navigation probe rather than weakening the assertion.
• Review agent pass: 0 must-fix, 1 should-fix (explicit apiMock.mockReset() in afterEach — vi.restoreAllMocks() stops resetting module-factory vi.fn() mocks in vitest 3), applied along with two nits (removed a redundant assertion-implied test, replaced an as Promise<never> cast with typed mockResolvedValueOnce queues).
• Full validation after fixes: unit suites green, finalize:web clean, E2E 174 pass / 12 skip.

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