Harden Docker Sandboxes recovery and support v0.42.1

[joecklau]

Summary

• Harden Docker Sandboxes control-plane recovery and lifecycle reconciliation with durable host-wide coordination, exact identity checks, and capacity-preserving retries.
• Reconcile historical sandbox identities only after independent exact-absence verification, and expose redacted provisioning failure causes.
• Support the inert default SSH-agent environment injected by sbx v0.42.1 while retaining rejection of actual agent endpoints and unexpected forwarding configuration.
• Export complete local-build SBOM evidence through ordinary files to avoid BuildKit attestation size limits, with bounded validation and artifact association.
• Add regression coverage and update operational and security documentation.

Validation

• Relevant Go test packages passed: internal/image, internal/pool, internal/provider/dockersandboxes, internal/prebuilt, and cmd/epar-prebuilt-publisher. Tests requiring local listeners were run outside the sandbox with a canonical temporary directory.
• Local Full arm64 template built and passed live admission on sbx v0.42.1.
• Live two-runner lifecycle verification exercised replacement and cleanup.
• Operator reports three successful CI jobs with runner recycling and replacement using the fixed local image.

Remaining verification and follow-ups

• Published prebuilt-image verification remains pending; the normal configuration uses full-latest, whose publication requires a Full schedule or explicit Full workflow dispatch.
• A startup host-trust refresh cancellation warning remains a separate follow-up; this PR does not claim to resolve it.
• Separate September 2–8 resource-exhaustion concerns are not claimed resolved by this change.
• Local configuration, credentials, and investigation evidence are excluded.

Includes commits 67e2b8a and 5ff0ec7. Targets develop; this PR does not merge or publish the stable image.