fix(#2388): gate shared requirement completion on sibling plans, revert on gaps (#2424)

* fix(#2388): gate shared-ID requirement marking and revert on gaps_found

Adds requirements.ready-ids (execute-plan.md's update_requirements step)
so a requirement ID declared by multiple plans in a phase only marks
Complete once every declaring plan has produced a SUMMARY.md, and
requirements.revert-phase (execute-phase.md's gaps_found branch) so a
gaps_found verdict reverts the phase's own prematurely-Complete IDs
before the gap report renders. Single-plan IDs still mark immediately.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* chore(#2388): regenerate fixtures + lint gate-prep

* fix(#2388): repair failing tests after gate verification

* chore(#2388): add changeset (#2424)

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

gsd-core/workflows/execute-phase.md

@@ -1439,9 +1439,10 @@ Verify-work will walk you through each item and mark the phase complete when all
 
 **If user acknowledges without reporting issues (including "ok", "noted", "ack", "got it", "approved", "done", "yes", "pass", or similar):** Stop. The phase remains pending. No further orchestrator action — wait for the user to run `/gsd:verify-work`.
 
-**If user reports issues now (before running verify-work):** Proceed to gap closure as currently implemented.
+**If user reports issues now:** Proceed to gap closure.
 
 **If gaps_found:**
+@~/.claude/gsd-core/references/execute-phase-requirement-revert.md
 ```
 ## ⚠ Phase {X}: {Name} — Gaps Found