Add Memory Gateway quality guardrails

[cayohollanda]

Summary

• add coverage-aware Memory Gateway checklist guardrails for multi-part prompts
• downgrade false-positive sufficient decisions to partial/insufficient when critical requested items are missing or current source-of-truth state is requested
• add prompt-aware retrieval boosting for notes that cover missing checklist items
• update docs, tests, changelog, and version to 0.12.0

Privacy

• no personal memory packs, session exports, benchmark HTML, or raw benchmark data are included in this PR
• only project source/docs/tests/package metadata are committed

Validation

• quick Pi smoke test with the local extension completed without crashing
• bun test test/unit.test.ts (fails before tests due existing @sinclair/typebox ESM export issue: Export named 'IsNumber' not found)
• npm run typecheck (attempted but timed out locally; this timeout was already observed before this change)