Adds a new Architecture Deep Dives section to AGENTS.md that documents complex flows in the codebase. This section is intended to be expanded over time as we document more areas of the app.

First entry: Job Execution on Local Providers

Documents the full flow when a job is queued for a local provider, covering:

1. Queueing — Router builds a ClusterConfig and enqueues via enqueue_local_launch()
2. Serialized worker — Background asyncio worker pulls items one at a time and launches via the provider instance
3. Local execution — LocalProvider.launch_cluster() creates a per-job uv venv and runs the command via subprocess.Popen
4. Error handling — Local providers use the queue worker's try/except; remote providers use tfl-remote-trap
5. Plugin harness — Subprocess entry point for plugin-based jobs (training, eval)