fix(mcp): reject empty anchor in MemoryFetch (#219)

[radimsem]

Closes #219.

What

HandleFetch passed input.Anchor straight to Engine.Fetch with no guard. An empty anchor produced a generic failed to fetch: ...node not found error that obscured the real problem (a missing required argument), hurting LLM tool-use recovery loops.

Change

• pkg/mcp/tools/fetch.go — add an empty-anchor guard immediately after defer d.logCall(...) (so the rejected call still logs), returning anchor is required. Mirrors the existing precedent in related.go, fetch_batch.go, and history.go.
• pkg/mcp/tools/tools_test.go — TestHandleFetch_EmptyAnchor, mirroring TestHandleHistory_EmptyAnchor. Asserts the anchor is required message and the absence of the generic failed to fetch error, so the test proves the guard short-circuits before Engine.Fetch rather than passing on the engine's own empty-ID error (the [enhancement] HandleWrite swallows GetNode errors with blank identifier — non-ErrNoRows DB failures silently fall into the create path #218 false-positive trap).

Notes

• Contract-neutral: anchor is already documented as the required node ID, so no skills/remind/ update is needed — consistent with the sibling guards.
• Verified: `make build`, `make test`, `make lint` (0 issues) all green; stash-the-fix confirms the new test fails without the guard.

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