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>

tests/fixtures/install-tree/opencode.json

@@ -159,6 +159,7 @@
   "gsd-core/references/execute-mvp-tdd.md",
   "gsd-core/references/execute-phase-between-wave-reset.md",
   "gsd-core/references/execute-phase-context-guard.md",
+  "gsd-core/references/execute-phase-requirement-revert.md",
   "gsd-core/references/execute-phase-wave-guard.md",
   "gsd-core/references/executor-examples.md",
   "gsd-core/references/few-shot-examples/plan-checker.md",