Runner error handling

[AndreiCravtov]

Runner error handling

Motivation

Runner failures were mostly surfaced as plain shutdown messages, which made root cause hard to spot from API errors or runner status.

This adds a MVP path for preserving runner crash context and attaching known stderr diagnostics to failure reports.

Changes

• Added RunnerTerminationError for Python exceptions raised inside runner bootstrap
• Changed runner bootstrap to send Event | RunnerTerminationError over the private runner channel
• Moved public RunnerFailed emission back into supervisor
• Added stderr-only RunnerDiagnosticCollector
• • Added known diagnostics for Metal GPU timeout, ring socket receive errno, and ring transport abort
• Added diagnostics to RunnerFailed and ErrorChunk
• Tweaked async process termination to join briefly before terminate/kill
• Updated tests/fixtures for new failure payload shape
• Added Ruff VS Code formatter settings

Why It Works

Runner child now reports raw-ish failure context to supervisor instead of publishing failed status directly.

Supervisor still owns process lifecycle, exit code/signal handling, in-flight task error chunks, and final runner status. Stderr diagnostics stay best effort and only known root-cause variants are surfaced.

Test Plan

Manual Testing

Hardware: remote runner logs from e16/e11/e4/e2

What you did:

• inspected live runner stderr logs
• used observed Metal GPU timeout and ring socket errors as initial diagnostic targets

Automated Testing

• nix flake check
• supervisor test covers error chunk + failed status emission
• plan lifecycle test updated for failed runner diagnostics
• type/lint checks cover new runner channel union