feat(T3.4): implement human-in-the-loop approval gates
- Add internal/approval package for workflow approval gates
- Implement ApprovalGate for gating workflow progression
- Implement ApprovalGateManager for managing multiple gates
- Gate status tracking: pending, approved, rejected, expired
- TTL-based gate expiration (auto-expire after timeout)
- Multiple approval tracking (configurable approval count)
- History tracking for all approval decisions
- Query by task, workflow, status
- Audit trail with decision reasons
- 16 approval tests, all passing
Test
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