Incident containment: real enforcement effects

## Description

`src/incident/containment.rs` line ~42 has a TODO: "Implement actual containment effects (Phase 7 T076-T080)". The containment primitives (FreezeHost, QuarantineWorkloadClass, BlockSubmitter, RevokeArtifact, DrainHostPool) create audit records but do not execute real enforcement.

## Requirements

- **FreezeHost**: Send SIGSTOP to all sandbox processes on target host, block new lease assignments
- **QuarantineWorkloadClass**: Update policy engine to reject jobs of the quarantined class
- **BlockSubmitter**: Add submitter to ban list, reject new submissions, cancel in-flight jobs
- **RevokeArtifact**: Remove artifact from approved registry, halt jobs using it
- **DrainHostPool**: Gracefully migrate all workloads off a set of hosts, block new assignments
- All actions must complete within 60 seconds (per spec)
- All actions produce immutable audit records with actor identity, justification, reversibility

## Success Criteria

- [ ] Each containment primitive executes its enforcement effect, not just logs
- [ ] FreezeHost stops all sandbox processes within 60 seconds
- [ ] QuarantineWorkloadClass causes policy engine to reject new jobs of that class
- [ ] BlockSubmitter prevents new submissions and cancels in-flight jobs
- [ ] RevokeArtifact halts jobs using the revoked artifact
- [ ] All actions are reversible (except RevokeArtifact which requires re-signing)
- [ ] Integration tests for each containment primitive
- [ ] `cargo test` passes

## Testing (Principle V)

- Start real sandbox, trigger FreezeHost, verify processes stopped
- Submit job of quarantined class → rejected
- Block submitter, verify in-flight job cancelled
- Revoke artifact, verify running job using it is halted