fix(mcp): validate anchor in MemoryHistory

[radimsem]

Closes #201.

Summary

HandleHistory was the only anchor-taking tool in pkg/mcp/tools/ without input validation: a mistyped ID fell through to GetDiffsForNode and rendered as "no history for <anchor>" — a confidently-wrong answer indistinguishable from a real-but-empty trail.

This adds the empty-anchor guard + node-existence check that sibling handlers (forget, pin) use, adapted to the tool's nature:

• Empty anchor → anchor is required (matches related.go's field-named message).
• GetDiffsForNode runs first; GetNode is probed only when the trail is empty, to distinguish a live-but-unchanged node (no history for X) from an unknown ID (node_id not found: X).

Why trail-first (the important bit)

A naive copy of the sibling preflight — GetNode before everything — is a functional regression. qSelectDiffsForNode reads FROM diffs WHERE node_id = ?, keyed independently of the nodes table, so a forgotten node (removed from nodes, rem/prior diffs retained) still has a trail that history must surface — that's how you roll back a delete by re-writing the old payload. An unconditional preflight would return node_id not found for exactly those nodes. Caught in review (Codex), fixed, and locked down with a regression test.

Input	Behavior
empty anchor	anchor is required
unknown ID (no diffs, not in nodes)	node_id not found: X — the bug, fixed
live node, no changes	no history for X — legit empty preserved
forgotten node (diffs present, not in nodes)	renders trail — regression avoided

Tests

• TestHandleHistory — happy path now seeds a real node.
• TestHandleHistory_EmptyAnchor, TestHandleHistory_MissingNode — the two new error paths.
• TestHandleHistory_DeletedNodeKeepsTrail — seeds a rem diff for a node absent from nodes, asserts the trail still renders.

Verification

make build · gofmt -l clean · make test (full suite, 212 MCP tests) · make lint 0 issues. Read-tool discipline preserved (no OpMu, no snapshot, no boost change). No public-skill update required (tool name/input/return shape unchanged).

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