feat(FN-1567): add mission contract assertions model types and APIs
- Add mission contract assertions model with types: ContractAssertion, ContractAssertionStatus, ContractAssertionType
- Add assertions table to schema v29 with migration from v28
- Implement assertion APIs: create, update, link/unlink to features, list, getWithContext
- Add rollup computation (assertion counts, pass rates) for missions and milestones
- Add many-to-many feature-to-assertion linking via featureAssertions table
- Add project memory documentation for assertion lifecycle and patterns
- Update schema version assertions in db.test.ts, run-audit.test.ts, and task-documents.test.ts
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