feat: MCP layer — BM25 search, embeddings tool, cross-repo tools + enable FTS5

Completes the rebase by adding the MCP handler layer:

1. Enable FTS5 in SQLite compile flags (-DSQLITE_ENABLE_FTS5).
   Without this, CREATE VIRTUAL TABLE USING fts5(...) silently creates
   a stub that fails on any query with 'no such module: fts5'.

2. Expose 'query' and 'sort_by' params in search_graph inputSchema.
   AI agents can now send natural language queries for BM25 ranked search
   instead of regex patterns only.

3. BM25 search path in handle_search_graph.
   When 'query' is provided, uses FTS5 MATCH with label-type structural
   boosting (Function/Method +10, Route +8, Class +5). Falls back to
   regex path when FTS5 is unavailable.

4. FTS5 backfill with contentless delete-all syntax.
   Contentless FTS5 tables (content='') require
   INSERT INTO table(table) VALUES('delete-all') instead of DELETE FROM.
   Falls back to plain names if cbm_camel_split is unavailable.

5. generate_embeddings MCP tool — manual trigger for embedding generation.

6. build_cross_repo_index MCP tool — builds unified _cross_repo.db.

7. trace_cross_repo MCP tool — cross-repo channel flow tracing.

8. Tool dispatch entries for all 3 new tools.

Tested: 'audio stream' on 713-node repo returns 28 ranked results
(useMicStream, startStream, stopStream) instead of 713 unranked.

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