[codex] Add search debounce feedback

[vku2018]

Summary

• Add a visible shared Spinner / "Searching..." status while the search input is still debouncing or a backend request is in flight.
• Send service search requests with an AbortSignal and guard completion by latest request/input so stale responses cannot replace newer results.
• Surface backend failures with role="alert", keep service detail links, and document the search feedback behavior in README.

Refs #65

Validation

• npm test -- src/app/search/page.test.tsx (13 passing)
• npx eslint src/app/search/page.tsx src/app/search/page.test.tsx
• git diff --check (passes with existing CRLF warnings only)
• npm run typecheck (blocked by pre-existing missing imports in src/app/agents/[agent]/page.tsx and src/app/docs/page.tsx)