Remove the retrieve_docs MCP tool; keep search_docs + get_doc_content

[tae2089]

What

retrieve_docs was search_docs (find relevant docs) + get_doc_content (read one) composed into a single call, plus opt-in scoring explain diagnostics agents rarely use. It overlapped the core doc-search surface. Drop it; callers compose search_docs → get_doc_content.

Changes

• Remove the retrieve_docs MCP tool: handler, DB helper, response DTO aliases, and the retrieve_docs-only doc-content readers (readIndexedDocContent/resolveIndexedDocPath) in handler_docs.go; registration in tools_docs.go.
• Collapse the shared retrieval engine now that the explain path is gone: FromDBWithOptions folds into FromDB, BuildDBResultWithOptions into BuildDBResult; delete the Options{Explain} type and the dead MatchedFields/ScoreDBFieldBreakdown helpers.
• Drop the four explain-only fields (field_scores, literal_score, expanded_terms, expansion_score) from ragindex.RetrieveResult.
• Sweep docs/skills to route doc retrieval through search_docs + get_doc_content.

MCP tools 26 → 25. Net −407 lines.

Investigation (why this cut)

retrieval (scoring engine) and ragindex (wiki tree model) are shared by search_docs, retrieve_docs, and the Wiki — so neither can be removed. The only retrieve_docs-unique code was the Explain diagnostics path (FromDBWithOptions, explain fields, ScoreDBFieldBreakdown). search_docs and Wiki both call the non-explain FromDB, so trimming the explain wrapper leaves them untouched.

Review

Independent adversarial review: faithful no-op refactor, no P0/P1/P2. The new FromDB body is the original FromDBWithOptions with opts threaded out and the if opts.Explain branch removed (that branch only set omitempty fields); every other step (candidate search, scan supplement, grouping, annotation batch, sorting, bounded content read) is byte-identical. MatchedFields/ScoreDBFieldBreakdown confirmed dead; the 4 explain fields had zero other consumers.

Verification

• go build + go vet clean; go test -tags fts5 ./... green.
• Real MCP server: tools/list returns 25 tools; retrieve_docs absent; search_docs and get_doc_content present.

Independent PR off main (parallel to #14 and the pending trim PRs).

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