feat: add Exa AI-powered search tool

[tgonzalezc5]

Summary

• Adds tinyloom.plugins.exa_search, a tinyloom plugin that registers a web_search tool backed by Exa's web search API.
• Exposes Exa's full feature surface to the agent: auto/neural/fast/deep/instant search types, text/highlights/summary content modes (combinable), domain and text filters, category filter, date ranges, and user_location.
• Cascading content extraction (highlights → summary → text) so each result renders the most useful snippet that came back, with text truncated to 1500 chars to keep tool output compact.
• Optional dependency: pulls exa-py>=2.0.0 only when installed via the [exa] extra. The plugin gracefully no-ops with a stderr warning if EXA_API_KEY is unset and returns a clear error if exa-py isn't installed.
• Picks up EXA_API_KEY from the environment (loaded by tinyloom's existing dotenv handling).

Slots in alongside the existing optional plugins (mcp, mask, hook_scripts) and follows the same style and registration conventions.

Usage

uv add 'tinyloom[exa]'
export EXA_API_KEY=...

# tinyloom.yaml
plugins:
  - tinyloom.plugins.exa_search

The agent can then call:

{
  "tool": "web_search",
  "input": {
    "query": "test-time compute scaling",
    "category": "research paper",
    "start_published_date": "2024-01-01",
    "num_results": 5
  }
}

Files changed

• tinyloom/plugins/exa_search.py — new plugin (registers web_search tool, sets x-exa-integration tracking header)
• tests/test_plugins.py — 14 new tests covering activation, response rendering, parameter passthrough, fallback content extraction, missing-SDK / missing-key / no-results / SDK-exception paths
• docs/exa-plugin.md — new docs page modeled on docs/mcp-plugin.md
• pyproject.toml — exa = ["exa-py>=2.0.0"] optional extra
• tinyloom.example.yaml — commented entry alongside existing plugins
• .env.example — EXA_API_KEY entry
• uv.lock — regenerated after adding the optional extra

Test plan

• uv run pytest tests/ -q — 233 passed (14 new)
• uv run ruff check tinyloom/plugins/exa_search.py tests/test_plugins.py — only E701/E401 warnings, matching the style used in tinyloom/plugins/todo.py, mcp.py, mask.py (one-line ifs and grouped imports per CONTRIBUTING.md style guide)
• Tool registration skipped (with stderr warning) when EXA_API_KEY is unset
• Clear error message when exa-py isn't installed
• Filters (include_domains, category, start_published_date, type, summary, …) pass through to the SDK
• Content fallback verified: highlights → summary → text → empty
• Manual smoke test against live Exa API (requires reviewer's EXA_API_KEY)

[shadowcodex]

Hey @tgonzalezc5 thanks for the PR. I'll take a look at it this week between conference meetings and see how we want to handle 1st party tools like this.

[tgonzalezc5]

dope sounds good, good luck with the conference!