fix(sandbox): pin cache helpers to local runtime image IDs

[yannickmonney]

Cache permission setup switched to the already-local runtime image in #4551. A configured mutable tag could then select a different root helper if its local mapping changed. Resolve and validate the local sha256 image ID first and run that exact image with --pull=never, --network none and /bin/chmod as the entrypoint. Missing or invalid identities refuse execution; missing-image warmup signaling and fresh-volume cleanup remain intact.

The operator README and existing automation row describe the same contract. New main incarnation tests and its registry dependency updater remain intact. The branch is rebased onto current main; the retention test’s fake Docker now answers the same local-image inspection prerequisite as the other volume fixtures.

Validation:

• Pinned Bun 1.4.2: 44 focused tests pass, 147 assertions, including current main’s package-cache retention tests. All seven new identity regression cases fail against the pre-fix implementation.
• Sandbox typecheck and type-aware lint; formatting, conflict, manual and documentation-link guards pass.
• Ordinary pre-commit and commitlint hooks pass. The repository SAST scan reports zero findings.
• The initial full bun run check used host Bun 1.3.10 and stopped at the unchanged E2E interrupted-body fixture. That case passes with pinned Bun 1.4.2; A later broad sandbox run encountered timeouts and did not complete; complete repository validation remains for native CI.

No real Docker daemon or provider was exercised; the tests drive bounded subprocess fixtures and composed session behavior.