docs(exec-plans): expand merge queue plan with Phase 2 spec

Rewrites the short decision record into a self-contained ExecPlan:
- Explicit Phase 2 steps for test.yml (merge_group trigger, detect-changes
  fix, lint-codeblocks condition) now that #7433 is resolved
- Behavioral acceptance criteria for both phases
- Rollout discoveries: Phase 1 first merge bypassed the queue;
  CircleCI build count unchanged (queue builds on throwaway branches)
- Branch-protection state snapshot and recovery / idempotence notes
- gh CLI one-liners to inspect protection state and verify queue usage

No decisions changed; facts and status carried forward from the
original record.

Author: jstirnaman