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

scripts/gen-plugin-skills.cjs

@@ -0,0 +1,117 @@
+#!/usr/bin/env node
+'use strict';
+
+/**
+ * gen-plugin-skills.cjs — generates skills/gsd-<stem>/SKILL.md from
+ * commands/gsd/*.md using convertClaudeCommandToClaudeSkill.
+ *
+ * Usage:
+ *   node scripts/gen-plugin-skills.cjs              # print summary to stdout
+ *   node scripts/gen-plugin-skills.cjs --write      # write skills/ dir
+ *   node scripts/gen-plugin-skills.cjs --check      # exit 1 if committed skills/ is stale
+ *
+ * #1596 Phase B-provide. The Claude Code plugin contract discovers skills from
+ * a skills/ directory (plugins-reference). GSD's source-of-truth commands live
+ * in commands/gsd/*.md (command frontmatter); this script converts each to
+ * skill format using the same convertClaudeCommandToClaudeSkill the file-copy
+ * installer uses, producing a build-generated skills/ dir that ships in the
+ * npm package and serves plugin-only installs.
+ *
+ * Depends on: gsd-core/bin/lib/runtime-artifact-conversion.cjs (compiled from
+ * src/runtime-artifact-conversion.cts by `npm run build:lib`). Must run AFTER
+ * build:lib in the build chain.
+ */
+
+const fs = require('node:fs');
+const path = require('node:path');
+const { ExitError, runMain } = require('./lib/cli-exit.cjs');
+
+const ROOT = path.resolve(__dirname, '..');
+const COMMANDS_DIR = path.join(ROOT, 'commands', 'gsd');
+const SKILLS_DIR = path.join(ROOT, 'skills');
+const CONVERSION_MODULE = path.join(ROOT, 'gsd-core', 'bin', 'lib', 'runtime-artifact-conversion.cjs');
+const PREFIX = 'gsd-';
+const RUNTIME = 'claude';
+
+function generateSkills(conversion) {
+  const cmdNames = conversion.readGsdCommandNames();
+  const files = fs.readdirSync(COMMANDS_DIR).filter(f => f.endsWith('.md'));
+  const results = [];
+  for (const file of files) {
+    const stem = file.slice(0, -3);
+    const skillName = PREFIX + stem;
+    const src = fs.readFileSync(path.join(COMMANDS_DIR, file), 'utf8');
+    const converted = conversion.convertClaudeCommandToClaudeSkill(src, skillName, RUNTIME, cmdNames, true);
+    results.push({ skillName, content: converted });
+  }
+  return results;
+}
+
+function main() {
+  const args = new Set(process.argv.slice(2));
+  const WRITE = args.has('--write');
+  const CHECK = args.has('--check');
+
+  if (!fs.existsSync(CONVERSION_MODULE)) {
+    throw new ExitError(
+      1,
+      `gen-plugin-skills: ${path.relative(ROOT, CONVERSION_MODULE)} not found.\n` +
+      'Run `npm run build:lib` first (this script depends on the compiled converter).'
+    );
+  }
+  const conversion = require(CONVERSION_MODULE);
+  const results = generateSkills(conversion);
+
+  if (WRITE) {
+    fs.rmSync(SKILLS_DIR, { recursive: true, force: true });
+    fs.mkdirSync(SKILLS_DIR, { recursive: true });
+    for (const { skillName, content } of results) {
+      const skillDir = path.join(SKILLS_DIR, skillName);
+      fs.mkdirSync(skillDir, { recursive: true });
+      fs.writeFileSync(path.join(skillDir, 'SKILL.md'), content);
+    }
+    process.stdout.write(`gen-plugin-skills: wrote ${results.length} skills to ${path.relative(ROOT, SKILLS_DIR)}/\n`);
+    return 0;
+  }
+
+  if (CHECK) {
+    if (!fs.existsSync(SKILLS_DIR)) {
+      throw new ExitError(1, 'gen-plugin-skills: skills/ missing. Run: npm run gen:plugin-skills -- --write');
+    }
+    let stale = 0;
+    const expectedNames = new Set(results.map(r => r.skillName));
+    for (const { skillName, content } of results) {
+      const skillMd = path.join(SKILLS_DIR, skillName, 'SKILL.md');
+      if (!fs.existsSync(skillMd)) {
+        process.stderr.write(`gen-plugin-skills: missing ${path.relative(ROOT, skillMd)}\n`);
+        stale++;
+        continue;
+      }
+      if (fs.readFileSync(skillMd, 'utf8') !== content) {
+        process.stderr.write(`gen-plugin-skills: stale ${path.relative(ROOT, skillMd)}\n`);
+        stale++;
+      }
+    }
+    const existingDirs = fs.readdirSync(SKILLS_DIR, { withFileTypes: true })
+      .filter(e => e.isDirectory() && e.name.startsWith(PREFIX));
+    for (const dir of existingDirs) {
+      if (!expectedNames.has(dir.name)) {
+        process.stderr.write(`gen-plugin-skills: stale (no source) ${path.relative(ROOT, path.join(SKILLS_DIR, dir.name))}\n`);
+        stale++;
+      }
+    }
+    if (stale > 0) {
+      throw new ExitError(1, `gen-plugin-skills: ${stale} stale skill(s). Run: npm run gen:plugin-skills -- --write`);
+    }
+    process.stdout.write(`gen-plugin-skills: ${results.length} skills up to date\n`);
+    return 0;
+  }
+
+  process.stdout.write(
+    `gen-plugin-skills: would write ${results.length} skills to ${path.relative(ROOT, SKILLS_DIR)}/\n` +
+    '  (use --write to generate, --check to verify staleness)\n'
+  );
+  return 0;
+}
+
+runMain(main);