feat: add Exa AI-powered search tool

[tgonzalezc5]

Summary

Adds kelvin.exa, a wasm_tool_v1 tool plugin that calls the Exa /search API for AI-powered web search. It mirrors the structure and ABI of the existing kelvin-websearch-plugin (Brave) so it slots into the same plugin lifecycle (make build / test / pack / install).

• New plugin at plugins/kelvin-exa-plugin/ (no_std cdylib, ~11 KiB WASM)
• Sandboxed: network_allow_hosts: ["api.exa.ai"], env_allow: ["EXA_API_KEY"]
• Exposes query, num_results, type (auto/neural/fast), category, include_domains / exclude_domains, start_published_date / end_published_date, livecrawl, and a summary flag
• Always requests highlights + a short text snippet; formats output with a highlights -> summary -> text fallback chain so results are immediately useful to the model
• Updates release/README.md to document the new plugin and EXA_API_KEY env var

Usage

// tool call arguments
{
  "query": "latest research on retrieval-augmented generation",
  "num_results": 5,
  "category": "research paper",
  "type": "neural",
  "summary": true
}

# Install / configure
cd plugins/kelvin-exa-plugin
make build
make test            # validates the manifest
make pack && make install

# Then in ~/.kelvinclaw/.env
EXA_API_KEY=<your-exa-key>

Files changed

• plugins/kelvin-exa-plugin/Cargo.toml, Cargo.lock — crate config (cdylib, no_std, wasm32 target)
• plugins/kelvin-exa-plugin/plugin.json — manifest with capabilities, scopes, and tool_input_schema
• plugins/kelvin-exa-plugin/src/lib.rs — guest implementation: input parsing, request build, response formatting
• plugins/kelvin-exa-plugin/build.sh, Makefile, .gitignore, README.md, payload/README.md — build/dev tooling matching the websearch plugin layout
• release/README.md — adds kelvin.exa row to the Tool Plugins table and EXA_API_KEY to the env var table

Test plan

• bash build.sh compiles the WASM cdylib successfully (~11 KiB, well under the 512 KiB module limit)
• scripts/kelvin-plugin-dev.sh test --manifest plugins/kelvin-exa-plugin/plugin.json validates the manifest and capability/runtime declarations (manifest ok: kelvin.exa@0.1.0 (wasm_tool_v1))
• cargo check --workspace (root) passes; the plugin uses an empty [workspace] to stay independent of the host workspace, matching the existing kelvin-websearch-plugin and kelvin-wiki-plugin layout
• Search-type, category, and livecrawl parameters are validated against strict allowlists in the guest before being forwarded to the API
• End-to-end live call against api.exa.ai/search with a real EXA_API_KEY (left to maintainers / a follow-up smoke run; no plugin-level unit tests since the plugin is no_std cdylib WASM, matching the other tool plugins in this repo)

Notes

• The plugin tags requests with an x-exa-integration: kelvinclaw header so usage attributable to KelvinClaw is observable on the Exa side, mirroring how other Exa integrations are tagged.
• I deliberately did not touch index.json or release/vendor/ — those carry pre-built tarballs for the public plugin index and seem to be regenerated as part of release tagging rather than per-PR. Happy to add an entry if maintainers prefer it bundled in this PR.