feat: add Exa AI-powered search tool

[tgonzalezc5]

Summary

Adds Exa — an AI-powered web search API built for LLMs and agents — as a new tool integration. Exa returns high-quality results via neural/semantic or keyword-like search, with on-demand content retrieval (full text, highlights, summaries) in a single request. It's a strong fit for content research, competitor discovery, link prospecting, news monitoring, and audience research workflows that marketing agents run.

What's added

• CLI (tools/clis/exa.js) — zero-dependency Node.js wrapper matching the existing CLI pattern (native fetch, env-var auth, JSON output, --dry-run with masked credentials). Supports:
  • search — /search with type (neural / fast / auto / deep-lite / deep / deep-reasoning / instant), category, domain + text filters, published/crawl date ranges, userLocation, and composable contents (text/highlights/summary can be requested together)
  • find-similar — /findSimilar to discover pages similar to a given URL
  • contents — /contents to fetch text/highlights/summary for one or more known URLs
• Integration guide (tools/integrations/exa.md) — auth, endpoints, common agent operations (competitor content discovery, find-similar, category-filtered news search, contents fetch), parameter reference, and linked skills
• Registry update (tools/REGISTRY.md) — new "AI Search" category + MCP-enabled list entry
• CLI README (tools/clis/README.md) — adds EXA_API_KEY env var and CLI listing

Usage

# Web search with highlights and truncated page text
EXA_API_KEY=... node tools/clis/exa.js search \
  --query "B2B SaaS onboarding teardowns" --num 10 \
  --text --max-chars 1000 --highlights

# Find pages similar to a competitor's pricing page
node tools/clis/exa.js find-similar \
  --url https://stripe.com/pricing --num 20 --summary \
  --summary-query "What pricing model and price points does this page use?"

# News in a category, filtered by date and location
node tools/clis/exa.js search \
  --query "DTC beauty brand raising Series A" --category news \
  --start-published 2024-06-01T00:00:00Z --user-location US

# Preview any request without hitting the API
node tools/clis/exa.js search --query test --dry-run

Files changed

• tools/clis/exa.js (new) — 130-line CLI wrapper
• tools/integrations/exa.md (new) — integration guide
• tools/REGISTRY.md — adds Exa row, new "AI Search" category section, MCP-enabled entry
• tools/clis/README.md — env var + CLI listing

Test plan

• Node syntax check passes (node --check tools/clis/exa.js)
• search builds correct /search body with filters, categories, and mixed contents fields (text + highlights + summary together) — verified via --dry-run
• find-similar builds correct /findSimilar body
• contents places text/highlights/summary at top level alongside urls (matches Exa API schema)
• Missing EXA_API_KEY exits with JSON error
• Missing required args (--query, --url, --urls) returns structured JSON error
• --dry-run masks the API key as *** (matches PR fix: harden CLI tools against credential leakage #253 credential-hardening pattern)
• x-exa-integration: marketingskills tracking header is sent on every request