feat: add Exa AI-powered search tool

[tgonzalezc5]

Summary

• Bundles the hosted Exa MCP server (https://mcp.exa.ai/mcp) in the Claude Code, Gemini CLI, and OpenCode configs, alongside the existing AWS Knowledge, Microsoft Learn, Google Developer Knowledge, Data Commons, and govreposcrape servers.
• Exposes three tools to ArcKit commands and agents: web_search_exa (clean, ready-to-use search results), web_fetch_exa (URL-to-markdown retrieval), web_search_advanced_exa (domain / date-range / highlight / subpage filters).
• Fills the gap the current bundle has for topics the cloud-vendor docs servers don't cover — vendor discovery, market research, regulatory/news context for SOBCs, and ad-hoc citations for research commands and the arckit-datascout agent.
• Works out of the box on Exa's free tier; new optional EXA_API_KEY entry in plugin.json userConfig lifts rate limits when set.

Usage

Once installed, the three Exa tools are available to any ArcKit command/agent that consumes MCP. Examples:

• web_search_exa(query: "UK public-sector CRM vendors 2025", num_results: 10) — quick search for vendor/market research
• web_fetch_exa(urls: ["https://example.com/whitepaper"]) — fetch a supplier whitepaper as markdown for citation extraction
• web_search_advanced_exa(query: "...", include_domains: ["gov.uk"], start_published_date: "2024-01-01") — scoped, date-filtered search for regulatory context

With the optional key:

# ~/.bashrc or ~/.zshrc
export EXA_API_KEY="your-key-from-dashboard.exa.ai"

Claude Code users can also set the key via the plugin's userConfig UI; the .mcp.json resolves ${user_config.EXA_API_KEY} automatically.

Files Changed

• arckit-claude/.mcp.json — register exa with x-api-key header
• arckit-gemini/gemini-extension.json — register exa
• arckit-opencode/opencode.json — register exa (enabled: true)
• arckit-claude/.claude-plugin/plugin.json — add EXA_API_KEY to userConfig
• docs/guides/mcp-servers.md + copies under arckit-opencode, arckit-codex, arckit-copilot, arckit-paperclip — document the server, its tools, and optional key setup
• README.md — list Exa in the two bundled-MCP summary lines and the "Why Claude Code?" comparison row
• CHANGELOG.md + arckit-claude/CHANGELOG.md — [Unreleased] entries

Test plan

• All edited JSON files parse (python3 -m json.tool clean on all 5)
• markdownlint-cli2 passes on all edited .md files (matches the repo's existing lint-markdown.yml workflow)
• Manual smoke test: install the plugin locally, confirm exa server shows up in /plugin and that web_search_exa returns results without a key
• Manual smoke test: set EXA_API_KEY, restart, confirm no error and that advanced features work