feat: add Exa AI-powered search tool

[tgonzalezc5]

Summary

Adds Exa as an alternative search provider alongside the existing Firecrawl integration. Exa is a neural search API designed for AI agents and exposes useful knobs for research workflows (search type, category filtering, domain filtering, date ranges) that often improve quality on technical or research-paper queries.

• Introduces a small SearchProvider abstraction in src/search.ts so additional providers can be added without touching deep-research.ts.
• Selectable via SEARCH_PROVIDER env var (firecrawl | exa). Firecrawl remains the default — existing setups are unchanged.
• Exposes Exa's search types, content modes (text + highlights + summary with cascading fallback), category filtering, domain filtering, and date-range filtering through optional environment variables.
• Adds unit tests using the repo's existing node:test framework.

Usage

In .env.local:

SEARCH_PROVIDER="exa"
EXA_API_KEY="your_exa_key"

# Optional tuning:
# EXA_SEARCH_TYPE="auto"          # auto | neural | fast | deep | deep-lite | deep-reasoning | instant
# EXA_CATEGORY="research paper"   # company | research paper | news | pdf | personal site | financial report | people
# EXA_INCLUDE_DOMAINS="arxiv.org,nature.com"
# EXA_EXCLUDE_DOMAINS="reddit.com"
# EXA_START_PUBLISHED_DATE="2024-01-01"
# EXA_END_PUBLISHED_DATE="2026-12-31"

Then npm start as usual.

Files changed

• src/search.ts — new provider abstraction (Firecrawl + Exa).
• src/search.test.ts — unit tests for response parsing, content fallback cascade, env-driven filters, and disabled state.
• src/deep-research.ts — switched the search call site from firecrawl.search(...) to the unified search(...). No behavioral change for default (Firecrawl) users.
• .env.example — documented SEARCH_PROVIDER and Exa env vars.
• README.md — added a short "Using Exa as the search provider" section.
• package.json / package-lock.json — adds exa-js dependency.

Test plan

• `npx tsc --noEmit` passes
• `npx tsx --test src/search.test.ts` — 14/14 passing
• Default Firecrawl path unchanged (no env var changes required)
• End-to-end run with a live Exa API key (requires reviewer's key)