Add test suite: 77 tests across 11 test files

Covers all major modules:
- Identity manager (validation, authorization, sessions)
- Policy engine (risk assessment, DSL enforcement)
- Policy DSL (parsing, evaluation, all verdicts)
- Approval gateway (request, approve, deny lifecycle)
- Blast radius analyzer (impact analysis, transitive deps)
- Merkle chain (append, verify, tamper detection, proofs)
- State manager (snapshots, history, diffing)
- Drift detector (provider, detection, severity classification)
- Filesystem adapter (shadow/real execution, all operations)
- Temporal branch engine (fork, evaluate, explore, scoring,
  merge requests with quorum, pairwise diffs, counterfactual
  analysis, pruning, sandbox mode, search, custom gate checks,
  budget enforcement, event emission)
- Integration tests (full lifecycle, DSL blocking, temporal
  exploration end-to-end)

Uses vitest as the test runner.