WIP: [codex] Add eval completion webhook support

[Curtis Galione (cjgalione)]

Summary

Adds an eval completion callback and devserver completion webhook support for JavaScript evals.

Changes

• Adds Eval(..., { onComplete }) and calls it once after eval results are produced and logs are flushed.
• Adds on_complete_webhook support to the devserver request schema.
• Adds completion webhook payload construction plus retry/backoff dispatch.
• Adds unit coverage for callback ordering and webhook dispatch behavior.

Validation

Not run locally in this cleanup pass; preserving WIP before machine wipe.