Scheduler/runner (start/stop/status) + run-event emission

[CodeBuildder]

Context

Orchestrates scenario lifecycle and emits bus events on start/stop. Parent: #24.

Acceptance Criteria

• POST /experiments starts a scenario and returns run_id
• GET /experiments/{run_id} returns status: pending|running|completed|failed
• DELETE /experiments/{run_id} stops the scenario early
• Start and stop events emitted to the bus as action events validating against event.schema.json

Dependencies

Blocked by: none | Blocks: none

Notes

No model tier.