Add durable pipeline status and error reporting

[anth-volk]

Goal

Add a durable pipeline status/error reporting surface for deployed Modal pipeline runs.

Scope

• Expose a protected pipeline status endpoint and Modal callable.
• Return run-level status, stage/substage manifests, missing expected manifest IDs, and durable error details.
• Persist redacted traceback records for pipeline and promotion failures.
• Keep status payloads bounded while retaining exact persisted error records on disk.
• Add AI guidance for diagnosing deployed pipeline failures through the status system.

Validation

The implementation should include focused unit tests, a deployed Modal seam test that skips without credentials, lint, and repository quality guards.