feat(agent): hybrid BM25 search_wiki tool for query/chat agent

[sebastianbraun25]

Problem

openkb query/openkb chat navigate the wiki purely via index.md one-line summaries plus
LLM-directed page reads (read_file, get_page_content). One-line summaries cannot surface facts
buried deep in a page body (a specific figure, a niche keyword, an exact term). As a wiki grows,
the agent increasingly misses pages whose index.md summary doesn't mention the exact term the
user asked about, even though the page body contains the answer.

Solution / Changes

• New module openkb/fulltext_index.py: a dependency-free BM25 (Robertson/Sparck-Jones) index
  over concepts/, entities/, and summaries/ wiki pages (PAGE_CONTENT_DIRS), rebuilt
  in-memory on construction — cheap at the wiki sizes this pattern targets, no on-disk cache or
  incremental update needed. Exposes WikiFullTextIndex.search(query, top_k) returning ranked
  SearchHit(path, title, score, snippet) results.
• openkb/agent/tools.py: new plain function search_wiki(query, wiki_root, top_k=5) formatting
  ranked hits as [[wikilink]] — title (score) / snippet text for the agent.
• openkb/agent/query.py: wires search_wiki in as a fourth tool (read_file,
  get_page_content, search_wiki, get_image) in build_query_agent — and therefore also in
  build_chat_agent, which extends the query agent's tools. Instructions updated with a new
  search-strategy step describing it as a hybrid, additive fallback: use alongside, not
  instead of, index.md navigation, so recall can only improve, never regress.
• No new dependency: BM25 is small enough in pure Python that a search-library dependency (e.g.
  Whoosh) isn't warranted, and openkb's dependencies are pinned exactly and vetted deliberately
  (see pyproject.toml).
• README: one paragraph documenting the hybrid retrieval behavior under "Query & Chat".
• Backward compatible: existing tools/instructions unchanged aside from the new step; no config,
  CLI flag, or schema changes.

Testing

• tests/test_fulltext_index.py (new): tokenization/BM25 ranking, PAGE_CONTENT_DIRS-only
  scoping, top_k, snippet extraction, title fallback, empty-index/empty-query edge cases.
• tests/test_agent_tools.py (new TestSearchWiki class): search_wiki formatting, no-match
  message, top_k.
• tests/test_query.py (updated): build_query_agent now exposes four tools including
  search_wiki.
• ruff check, ruff format --check, mypy openkb, pytest all green (pre-existing,
  environment-specific failures unrelated to this change are unaffected).

Issues

Resolves #233