README: change workflow says 12-step but code has 13 steps

## Bug

The README references the `pdd change` workflow as "12-step" in multiple places, but the actual implementation in `agentic_change_orchestrator.py` has 13 steps.

## Evidence

- `agentic_change_orchestrator.py:2` defines 13 steps
- README mentions "12-step" at lines 28, 573, 1919

## Fix

Update all README references from "12-step" to "13-step" for the change workflow.