test: add integration tests for MCP server tools (closes #5)

[clawtom]

Closes #5.

Adds tests/test_server_integration.py covering the three MCP tools from the issue: vault_stats, vault_prediction_errors, and vault_search.

Approach

The tests call tool functions directly as plain Python callables — no stdio or network transport needed. Each test patches neurostack.schema.DB_PATH to point to a hermetic temp-file database. This works because each tool imports DB_PATH lazily inside its function body, so the patch is visible at call time.

A server_db fixture creates a temp DB via get_db() (which runs all schema migrations) and inserts two notes, two chunks, and one prediction error — enough for all tools to return non-trivial results.

Coverage

• TestVaultStats (5 tests): valid JSON, required keys, counts match test data, stale-summary detection, empty vault
• TestVaultPredictionErrors (6 tests): valid JSON, required keys, returns inserted error, error-type filter, resolve clears error, empty vault
• TestVaultSearch (5 tests): valid JSON, keyword FTS finds matching note, result fields, no-match returns empty list, top_k limit

vault_search uses mode="keyword" (pure FTS5) throughout — no Ollama embedding service needed, consistent with the issue requirement.

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