PM planning — decompose ARCH into tasks

The pm agent's job is to take the architect's ARCH document and produce 2 things:

1. A PLAN-<feature>.md for humans
2. A sequence of bd create tasks for senior-dev to claim

The plan is good if a fresh senior-dev (no prior context) can pick up the bd tasks and ship without coming back for clarification.

The decomposition rules

Rule 1. Tasks ≤ 4 hours