Remote Triggers: add execution history and run status

Problem

Remote Triggers (/v1/code/triggers) have no way to check whether a scheduled trigger actually fired, succeeded, or failed. The API only exposes list, get, create, update, and run actions. After a trigger's next_run_at passes, there is no feedback on what happened.

Current experience

1. Set up a recurring remote trigger with a cron schedule
2. Wait for next_run_at to pass
3. No way to know if it fired, is still running, succeeded, or failed
4. Only signal is whether the side effect (in our case, a Discord webhook post) shows up

The run action (manual fire) also returns HTTP 500 consistently, so there's no reliable way to test a trigger either.

Requested features

1. Execution history: GET /v1/code/triggers/{id}/runs returning recent executions with status (pending, running, completed, failed), start/end timestamps, and error details if failed
2. Run status on GET: Include last_run_at, last_run_status, and last_run_error fields in the trigger object returned by get/list
3. Fix manual run endpoint: POST /v1/code/triggers/{id}/run currently returns HTTP 500. This should work for testing triggers without waiting for the cron schedule.
4. Execution logs (nice to have): ability to retrieve stdout/stderr or a summary of what the headless session did

Context

Using remote triggers for automated weekly monitoring jobs that post to Discord. Without execution visibility, debugging a silent failure requires waiting for the next cron window and hoping the side effect appears.