Plan Runner

Execute tasks from a structured plan file (any `*_PLAN.md` in the `plans/` folder) one task at a time, dispatching each to a worker sub-agent at the recommended tier. Also drives the test → investigate → fix → retest loop for test-focused tasks and verification gates, and enforces rollback safety for deploy tasks. Checks `plans/PLANS_REGISTRY.json` to skip completed plans. Use whenever the user says "run next task", "next task", "continue the plan", "what's next", "execute task X.X", "do task X.X", "run regression tests", "run the test plan", "run feature X tests", "test → fix until green", "regression check for {feature}", or names a specific task number. Also triggers o…