#8 API-3: Decisions REST API

Implement /v1/decisions create/read. Handle idempotency, context hash, and policy snapshotting. Development Workflow: 1. Read ticket 2. Mark it as in-progress (`issuedb-cli update <ID> -s in-progress`) 3. Write TDD test 4. pytest (red) 5. Implement 6. pytest (green) 7. Iterate and fix all issues 8. Quality check and cross-check with spec, audit 9. Verdict for closure 10. Close the ticket (`issuedb-cli update <ID> -s closed`) 11. Go to next ticket