[TheGreatAxios]

Closes CL-11845 (https://linear.app/abklabs/issue/CL-11845/stop-swallowing-errors-in-the-hubs-agent-runtime-paths).

Changes

Error handling in the hub's live-app host, runner and proxy, sidecar spawn and provisioning, build package layout, coordinator startup, resume and upgrade, mail, artifacts and run stream:

• Rewritten:
  • .catch(() => undefined) chains become async/await
  • IIFEs become named functions
  • empty and bare catches go away
• Expected failures are checked for, or caught by type or code (ENOENT, SyntaxError, a refused fetch), and everything else is rethrown.
• Best-effort cleanup logs the error with errorCode and requestErrorMetadata.
• New helpers: lib/json.ts (readJson, parseJson, undefined only on a syntax error) and errorCode in request-error.ts, which logs a thrown error's code and never its message or paths.
• Live-app host and proxy:
  • The host's stderr lines, which reach the app log the person sees, carry a fixed phrase and the error code, never a path or raw message.
  • The live-app proxy's server keeps answering 502 when a request fails.
  • mirrored.json is written atomically, and a corrupt one is mirrored again from scratch.
  • A failing file watch now degrades to a single sync, with a log line.

The rest of builds/, the interface and the hub's other areas are separate issues.

Verification

• Lint, format and typecheck pass. Locally, 65 of 66 tests pass. The one failure is a 5-second timeout in packaged.test.ts, and it fails the same way on unchanged main under load.
• Triage's local check passes all code-pattern rules on the change.
• An independent review found five issues. All are fixed or out of scope by narrowing the issue: paths in the person-visible log, missing error codes, a raw 500 from the proxy, a changed status code, and the unconverted builds/ folder.
• e2e runs in CI.

[linear-code]

CL-11845

[corbits-triage]

Triage: In progress

Triage is waiting for the decision model's answer before it decides. This comment shows the verdict once it has.