feat: add DocsGPT semantic search with backwards-compatible fallback

[critesjosh]

Summary

• Adds DocsGPT as an optional semantic search backend for aztec_search_docs and aztec_lookup_error, activated when API_KEY is set
• Preserves full backwards compatibility: schema always advertises section/maxResults, and DocsGPT errors gracefully fall back to ripgrep over local docs
• aztec_lookup_error gains a semantic fallback that searches documentation when the static error catalog has no matches

Review feedback addressed

• [P1] Schema regression: section and maxResults are always present in the advertised schema regardless of DocsGPT config. maxResults maps to chunks in semantic mode (chunks ?? maxResults ?? 5). Schema descriptions clarify that section applies to local fallback only.
• [P2] No fallback on DocsGPT failure: DocsGPT errors now fall through to ripgrep search instead of returning failure immediately. If local docs are cloned, the search still works; if not, the user gets the standard "run aztec_sync_repos" message.

Test plan

• tsc --noEmit passes
• vitest run tests/tools/search.test.ts — 19 tests pass, including:
  • Semantic results returned from DocsGPT client
  • chunks parameter respected
  • maxResults used as fallback for chunks in semantic mode
  • chunks takes precedence over maxResults when both provided
  • DocsGPT error falls back to ripgrep when local docs exist
  • DocsGPT error returns not-cloned message when no local docs
• vitest run tests/utils/error-lookup.test.ts — 15 tests pass

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