MCP: add an ask tool that returns a grounded, cited answer

[claude]

🤖 Agent-Generated PR

Closes #12
Agent Used: Claude Code
Complexity: medium

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Summary

Adds an ask MCP tool that runs retrieve→synthesize and returns a structured JSON response with answer (grounded text) and citations (path:line_start-line_end references), giving MCP clients (Claude Desktop, Cursor) a direct "what did I decide about X" answer instead of raw search chunks.

Implementation Notes

• mcpAsk is a standalone helper that accepts a synth.Client parameter for testability — the closure in runMCP builds the live client via answerClient(cfg) and checks available before calling through, surfacing a clear error when synthesis is unavailable (local_only without Ollama, missing API key, etc.).
• Returns {"answer": "...", "citations": ["path:line-line", ...]}. When no chunks are found the synth client is not called — returns {"answer": "No relevant notes found.", "citations": []} instead, avoiding an ungrounded hallucinated answer.
• since, tag, project, and k filters match the existing search tool so callers can scope the retrieval window.
• No new dependencies. Fully reuses searchChunks, answerQuery, and answerClient from search.go.

Changes

• cmd/mcp.go: added askInput and askResponse structs, registered ask tool in runMCP, implemented mcpAsk helper
• cmd/mcp_test.go: four table-driven tests covering happy path, empty query error, synth error propagation, and no-results short-circuit

Testing

Automated:

• Unit tests added/updated
• Integration tests added/updated
• All existing tests pass (make test — all 11 packages green)

Manual testing performed:

• Traced the happy path: searchChunks → answerQuery (fake client) → JSON marshal → citations populated
• Traced no-results path: synth client not called, empty citations returned
• Traced unavailable-synth path: answerClient returns available=false, error returned before mcpAsk is called

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Review Checklist

Correctness

• Code actually solves the issue
• Acceptance criteria are met
• Edge cases handled

Code Quality

• Follows existing codebase patterns
• Follows project coding standards
• Naming conventions consistent
• No unnecessary complexity
• Comments where needed (but code is self-documenting)

Security

• Input validation present
• No exposed secrets or credentials
• Data properly sanitized/escaped
• Authentication/authorization checks where needed

Performance

• No obvious N+1 queries
• Reasonable algorithmic complexity
• Caching used where appropriate
• No blocking operations in request lifecycle

Tests

• Tests actually test behavior (not just coverage)
• Edge cases covered
• Failure modes tested

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Reviewer Notes

The synth.Client injection pattern (passing the fake directly into mcpAsk) keeps the function pure and avoids any config/env inspection in tests, consistent with the approach used throughout cmd/answer_test.go.

[claude]

Retro: no durable lesson (merged clean / one-off).