fix: index sibling-project docs in site search

[hivecommons-hive]

Summary

• /api/search now builds its index from buildPageMap(projectId) for every project in PROJECTS, so sibling-project pages (pluk, dibs, hotshot, rationguard, promptargs, spektacular) are searchable and linked at /docs/<project>/<route>.
• Hive + general-section results/URLs are unchanged; shared general-section pages are indexed once (only hive probes the shared root).
• The index is built lazily and memoized at module level; q is capped at 200 characters.
• Sibling files resolve via docs/content/<project>/ (same as getContentPath), which keeps the existing search-route test mocks of page-map working unchanged.
• Added search-route.sibling-projects.test.ts and a changelog fragment.

Acceptance

• Searching an H1 from a sibling project page returns it with a /docs/<project>/... URL.
• Existing hive + community behavior unchanged.
• Route test covers a sibling-project file.

Closes #262

— hive: agent=scanner backend=copilot model=claude-fable-5.1 copilot=1.0.88

[kubestellar-prow]

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[hivecommons-hive]

hive/awaiting-ci: CI pending — sweep will merge when green.

---

🐝 Hive Agent: scanner | Instance: hosted-available-oke-11-placeholder-r05x | SHA: unknown

— hive: agent=scanner backend=copilot model=claude-fable-5.1 copilot=1.0.88

[hivecommons-hive]

hive/awaiting-ci: CI pending — sweep will merge when green.

---

🐝 Hive Agent: scanner | Instance: hosted-available-oke-11-placeholder-r05x | SHA: 9adc513

— hive: agent=scanner backend=copilot model=claude-fable-5.1 copilot=1.0.88