fix(gen-skill-docs): quote YAML inline scalars containing '...' (Bun strict parser breaks on bare ellipsis)

A bare ... inside a plain YAML scalar is a document-end marker that strict
YAML parsers (Bun.YAML among them) reject mid-scalar. catalog-trim truncation
appends '...' to any description whose lead exceeds 200 chars, so any
truncated description would generate a SKILL.md with unparseable frontmatter.
Add the ellipsis test to toYamlInlineScalar's needsQuote so such scalars are
emitted double-quoted, plus unit coverage for the quoting rules.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/catalog-trim.test.ts

@@ -23,8 +23,37 @@ import {
   buildTrimmedDescription,
   buildWhenToInvokeSection,
   applyCatalogTrim,
+  toYamlInlineScalar,
 } from '../scripts/gen-skill-docs';
 
+describe('toYamlInlineScalar', () => {
+  const parses = (out: string) => Bun.YAML.parse(`d: ${out}`);
+
+  test("scalar containing '...' (YAML document-end marker) is quoted and round-trips", () => {
+    const out = toYamlInlineScalar('Truncated lead ends with... more (gstack)');
+    expect(out.startsWith('"')).toBe(true);
+    expect((parses(out) as { d: string }).d).toContain('...');
+  });
+
+  test("interior ': ' is quoted (nested-mapping ambiguity, #1778)", () => {
+    const out = toYamlInlineScalar('Ship workflow: detect and merge');
+    expect(out.startsWith('"')).toBe(true);
+    expect((parses(out) as { d: string }).d).toBe('Ship workflow: detect and merge');
+  });
+
+  test('plain safe scalar passes through unquoted', () => {
+    expect(toYamlInlineScalar('Simple description here')).toBe('Simple description here');
+  });
+
+  test('leading YAML indicator char is quoted', () => {
+    expect(toYamlInlineScalar('- leading dash').startsWith('"')).toBe(true);
+  });
+
+  test('trailing whitespace is quoted', () => {
+    expect(toYamlInlineScalar('has trailing space ').startsWith('"')).toBe(true);
+  });
+});
+
 describe('splitCatalogDescription', () => {
   test('extracts lead sentence + routing prose from simple multi-line description', () => {
     const desc =