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

.changeset/rapid-bears-hum.md

@@ -0,0 +1,5 @@
+---
+type: Added
+pr: 0
+---
+**Plugin installs now expose GSD skills** — when GSD is installed as a Claude Code plugin (`claude plugin install`), its skills are available via `gsd-core:<skill>` the native way. Previously, plugin-only installs lacked the skill surface because `bin/install.js` never ran; agents that preload `global:gsd-core:<skill>` (PR #1261) now resolve against plugin-provided skills. (#1596)