feat(#1596): ship GSD skills via .claude-plugin/plugin.json

Phase B-provide of epic #1258. Adds a build-generated skills/ dir +
a skills manifest field so plugin-installed GSD exposes gsd-core:<skill>
the native Claude Code way. Closes the gap where plugin-only installs
lacked the skill surface because bin/install.js never ran.

- scripts/gen-plugin-skills.cjs: build step converting commands/gsd/*.md
  to skills/gsd-<stem>/SKILL.md via convertClaudeCommandToClaudeSkill
- .claude-plugin/plugin.json: add "skills": "./skills/"
- package.json: add skills to files, gen:plugin-skills to build chain
- tests/issue-766-plugin-manifest.test.cjs: Section H conformance
  (manifest field + dir + frontmatter + count parity) + C2 skills symlink
- docs/adr/766-*.md: dated amendment adding skills surface row
- .changeset/rapid-bears-hum.md: type Added
- skills/: 69 generated gsd-<stem>/SKILL.md files (build-committed)

Closes #1596

package.json

@@ -10,6 +10,7 @@
   "files": [
     "bin",
     "commands",
+    "skills",
     "gsd-core",
     "assets",
     "agents",
@@ -78,13 +79,14 @@
     "check:alias-drift": "node scripts/check-alias-drift.cjs",
     "check:identity-drift": "node scripts/lint-package-identity-drift.cjs",
     "check:integrity": "node scripts/check-npm-integrity.cjs",
-    "build": "npm run generate:identity && npm run build:lib && npm run gen:loop-host-contract && npm run gen:capability-registry && npm run build:hooks",
+    "build": "npm run generate:identity && npm run build:lib && npm run gen:plugin-skills && npm run gen:loop-host-contract && npm run gen:capability-registry && npm run build:hooks",
     "build:hooks": "node scripts/build-hooks.js",
     "build:lib": "tsc -p tsconfig.build.json",
     "generate:identity": "node scripts/generate-package-identity.cjs",
     "gen:loop-host-contract": "node scripts/gen-loop-host-contract.cjs --write",
+    "gen:plugin-skills": "node scripts/gen-plugin-skills.cjs --write",
     "gen:capability-registry": "node scripts/gen-capability-registry.cjs --write",
-    "prepack": "npm run build:lib",
+    "prepack": "npm run build:lib && npm run gen:plugin-skills",
     "prepare": "npm run build:lib",
     "version": "node scripts/sync-manifest-versions.cjs --stage && node scripts/gen-capability-registry.cjs --write && git add gsd-core/bin/lib/capability-registry.cjs",
     "prepublishOnly": "npm run build:lib && npm run build:hooks",