Webhooks - new execution field

[matthewlouisbrockman]

Webhooks now return data about the execution to assist with bookkeeping.

- `started_at` - UTC RFC3339 timestamp when the current sandbox execution started
- `vcpu_count` - Number of vCPUs assigned to the sandbox
- `memory_mb` - Memory assigned to the sandbox in MB
- `execution_time` - Sandbox runtime in seconds

Will be in next orchestrator release, do not merge before then.

---

Note

Low Risk
Documentation-only change; no runtime or API logic is modified, with minimal risk beyond potential spec mismatch if the backend rollout differs.

Overview
Updates sandbox lifecycle webhook documentation to reflect a new payload format: bumps the example version from v1 to v2 and adds an eventData.execution object with runtime details.

Documents when eventData.execution is present (on sandbox.lifecycle.killed and sandbox.lifecycle.paused) and defines its fields (started_at, vcpu_count, memory_mb, execution_time).

^{Written by Cursor Bugbot for commit cb9f679. This will update automatically on new commits. Configure here.}