fix(gen-skill-docs): quote frontmatter descriptions with interior colons (#1778)

Generated SKILL.md frontmatter emitted the catalog-trimmed description: as a
plain YAML scalar. A description with an interior ": " (e.g. "Ship workflow:
detect...") parses as a nested mapping under strict YAML loaders, so Codex/OpenAI
skill loading rejected those skills.

applyCatalogTrim now routes the value through toYamlInlineScalar, which quotes
(via JSON.stringify) only when a plain scalar would be invalid — interior ": ",
inline " #", leading indicator char, or surrounding whitespace. Strings that are
already valid plain scalars pass through unchanged to keep regen diffs small.

The frontmatter test now parses every generated block (Claude + Codex hosts) with
Bun.YAML.parse instead of string-checking that name:/description: substrings exist,
so the regression can't reappear. Runs under `bun test` (already in CI).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/gen-skill-docs.test.ts

@@ -155,12 +155,39 @@ describe('gen-skill-docs', () => {
     }
   });
 
-  test('every generated SKILL.md has valid YAML frontmatter', () => {
+  // #1778: strict YAML parsers (Codex/OpenAI skill loading) reject frontmatter
+  // whose plain `description:` scalar contains an interior ": " (read as a nested
+  // mapping). Parse EVERY generated frontmatter block with a strict YAML parser,
+  // not just string-check that name:/description: exist.
+  function frontmatterBlock(content: string): string {
+    expect(content.startsWith('---\n')).toBe(true);
+    const end = content.indexOf('\n---', 4);
+    expect(end).toBeGreaterThan(0);
+    return content.slice(4, end);
+  }
+
+  test('every generated SKILL.md frontmatter parses as strict YAML', () => {
     for (const skill of CLAUDE_GENERATED_SKILLS) {
       const content = fs.readFileSync(path.join(ROOT, skill.dir, 'SKILL.md'), 'utf-8');
-      expect(content.startsWith('---\n')).toBe(true);
-      expect(content).toContain('name:');
-      expect(content).toContain('description:');
+      const fm = frontmatterBlock(content);
+      let parsed: any;
+      expect(() => { parsed = Bun.YAML.parse(fm); },
+        `frontmatter for ${skill.dir} must be valid YAML`).not.toThrow();
+      expect(typeof parsed?.name).toBe('string');
+      expect(typeof parsed?.description).toBe('string');
+    }
+  });
+
+  test('every generated Codex (.agents/skills) frontmatter parses as strict YAML', () => {
+    const agentsDir = path.join(ROOT, '.agents', 'skills');
+    if (!fs.existsSync(agentsDir)) return; // skip if external hosts not generated
+    for (const entry of fs.readdirSync(agentsDir, { withFileTypes: true })) {
+      if (!entry.isDirectory()) continue;
+      const mdPath = path.join(agentsDir, entry.name, 'SKILL.md');
+      if (!fs.existsSync(mdPath)) continue;
+      const fm = frontmatterBlock(fs.readFileSync(mdPath, 'utf-8'));
+      expect(() => Bun.YAML.parse(fm),
+        `Codex frontmatter for ${entry.name} must be valid YAML`).not.toThrow();
     }
   });