fix(sandbox): replace lifetime exec cap with retry deadlines

[drew]

Summary

Long-lived sandboxes stop accepting new execs after 4,096 request IDs. Replace that lifetime cap with a 30-second admission and recovery deadline so fresh execs can continue while expired requests fail without running a command twice.

Related Issue

Closes #4103

Changes

• Carry an absolute exec expiration time in the request envelope and include it in the payload digest. Transport retries preserve the same ID, payload, and deadline.
• Retain request IDs until expiration, then reclaim them as new exec requests arrive. Reject expired requests before starting or reattaching a process, and prevent clock rollback from resurrecting discarded IDs.
• Preserve existing process retention limits and allow admitted commands to run beyond the recovery deadline. Report an uncertain execution outcome when startup recovery expires.
• Update the crate protocol notes. This private protocol change requires updating the supervisor and sandbox runtime together; their wall clocks must be synchronized.

Testing

• Checks appropriate to the affected code and behavior pass: cargo clippy -p openshell-sandbox-backend -p openshell-sandbox --all-targets -- -D warnings, Rust formatting, and git diff --check.
• Unit tests added/updated: 233 tests pass with cargo test -p openshell-sandbox-backend -p openshell-sandbox --lib. Coverage includes 10,000 admitted IDs, expiry and reclamation, delayed first attempts, missing deadlines, clock rollback, and digest-bound deadlines. The real boundary process test also launches and recovers an exec after 4,096 earlier IDs.
• E2E tests added/updated: four Docker exec tests pass using supervisor and sandbox images built from this checkout. They cover large output, piped stdin, environment propagation, and a 31-second command followed by a fresh exec in the same sandbox.
• mise run docs passes with no errors and three warnings.
• The repository pre-commit hook (mise run pre-commit) passes, including workspace Clippy, formatting, license, Markdown, Helm, protobuf, Python, and TypeScript checks.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated: private exec protocol and deployment compatibility documented in the crate README.

[github-actions]

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[purp]

LGTM 🚢