test: add name patching + ship idempotency tests (#620, #649)

- 4 unit tests for name: patching in relink.test.ts (prefix on/off,
  gstack-upgrade not double-prefixed, no-frontmatter no-op)
- 2 tests for gen-skill-docs prefix warning
- 1 E2E test for ship idempotency (periodic tier)
- Updated setupMockInstall to write SKILL.md with proper frontmatter
- Added ship-idempotency touchfiles + tier classification

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/helpers/touchfiles.ts

@@ -82,6 +82,7 @@ export const E2E_TOUCHFILES: Record<string, string[]> = {
   'review-dashboard-via': ['ship/**', 'scripts/resolvers/review.ts', 'codex/**', 'autoplan/**', 'land-and-deploy/**'],
   'ship-plan-completion': ['ship/**', 'scripts/gen-skill-docs.ts'],
   'ship-plan-verification': ['ship/**', 'scripts/gen-skill-docs.ts'],
+  'ship-idempotency':      ['ship/**', 'scripts/resolvers/utility.ts'],
 
   // Retro
   'retro':             ['retro/**'],
@@ -122,6 +123,7 @@ export const E2E_TOUCHFILES: Record<string, string[]> = {
   // Plan completion audit + verification
   'ship-plan-completion': ['ship/**', 'scripts/gen-skill-docs.ts'],
   'ship-plan-verification': ['ship/**', 'qa-only/**', 'scripts/gen-skill-docs.ts'],
+  'ship-idempotency':       ['ship/**', 'scripts/resolvers/utility.ts'],
   'review-plan-completion': ['review/**', 'scripts/gen-skill-docs.ts'],
 
   // Design
@@ -228,6 +230,7 @@ export const E2E_TIERS: Record<string, 'gate' | 'periodic'> = {
   'ship-triage': 'gate',
   'ship-plan-completion': 'gate',
   'ship-plan-verification': 'gate',
+  'ship-idempotency': 'periodic',
 
   // Retro — gate for cheap branch detection, periodic for full Opus retro
   'retro': 'periodic',