M1.1: Search and fetch tools with tests
- tavily_search(): Tavily API wrapper returning SearchResult dataclasses
with content hashing (raw_content preferred, falls back to summary)
- fetch_url(): async URL fetch with HTML text extraction, content hashing,
and graceful error handling (timeout, HTTP errors, connection errors)
- _extract_text(): simple HTML → clean text (strip scripts/styles/tags,
decode entities, collapse whitespace)
- _sha256(): SHA-256 content hashing with 'sha256:' prefix for traces
18 tests: hashing, HTML extraction, mocked Tavily search, mocked async
fetch (success, timeout, HTTP error, hash consistency).
Refs: archeious/marchwarden#1
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
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