feat: add Exa AI-powered search tool

[tgonzalezc5]

Summary

Adds Exa as a fifth search adapter, alongside DuckDuckGo, SearXNG, Brave, and Tavily. Exa is a neural-search API tuned for long, intent-rich queries — the shape the planner's sub-queries and the critic loop's gap-filling follow-ups tend to take.

• New adapter ExaSearch in src/search/exa.ts (~50 lines, no new runtime deps; uses raw fetch matching the Tavily / Brave pattern)
• Wired into resolveSearchAdapter in src/search.ts behind --search=exa and DEEPDIVE_EXA_KEY
• Snippet cascade: highlights → text → summary → "" so the adapter degrades gracefully across content modes
• Pure mapping function mapExaResults extracted for testability per the project's "pure decision functions" principle
• Includes x-exa-integration: deepdive header so Exa can attribute usage from this integration
• README adapter table + flag table + sovereignty paragraph updated; --help output and env var list updated; deepdive doctor picks up the new adapter automatically via resolveSearchAdapter

Usage

export DEEPDIVE_EXA_KEY=...
deepdive "how does claude's rate limiter work" --search=exa --deep --verbose --out=report.md

Files changed

• src/search/exa.ts (new) — adapter + pure mapping function
• src/search.ts — register exa case in resolveSearchAdapter
• src/cli.ts — adapter list in --help, env var in usage footer
• README.md — sovereignty paragraph, flag row, adapter row
• test/exa-adapter.test.mjs (new) — 14 tests covering parsing, snippet fallback cascade, resolver wiring, request shape (headers, body, numResults cap), and HTTP error handling

Test plan

• npm run typecheck — passes
• npm run build — passes
• npm test — 212/212 pass (14 new)
• Snippet fallback verified across all four states (highlights / text / summary / none)
• Adapter throws clearly when DEEPDIVE_EXA_KEY is unset
• Request body verified to send x-api-key and x-exa-integration: deepdive headers
• Live API smoke test against api.exa.ai (requires a key; tested locally on my end with deepdive "..." --search=exa --verbose, leaving this unchecked since CI won't have a key)

[askalf]

Thanks for this PR, @tgonzalezc5 — genuinely high-quality contribution. The pure mapExaResults extraction, the snippet-cascade fallback, the typed response interface, and the integration-attribution header all show real care. Exa was on the wishlist; you put it in better than it would've shipped if I'd written it. Appreciate it 🙏