refactor: extract processExternalHost() shared helper for multi-host generation

Refactor the Codex-specific output routing block in gen-skill-docs.ts into
a shared processExternalHost() function. Both Codex and future external hosts
(Factory Droid) will use this helper for output routing, symlink loop detection,
frontmatter transformation, path rewrites, and metadata generation.

- Rename codexSkillName() to externalSkillName() everywhere
- Extract ExternalHostConfig interface with per-host settings
- Codex output is byte-identical (verified via --dry-run)
- Skip /codex skill for all non-Claude hosts (not just codex)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

test/gen-skill-docs.test.ts

@@ -1318,7 +1318,7 @@ describe('Codex generation (--host codex)', () => {
     expect(content).toContain('allow_implicit_invocation: true');
   });
 
-  test('codexSkillName mapping: root is gstack, others are gstack-{dir}', () => {
+  test('externalSkillName mapping: root is gstack, others are gstack-{dir}', () => {
     // Root → gstack
     expect(fs.existsSync(path.join(AGENTS_DIR, 'gstack', 'SKILL.md'))).toBe(true);
     // Subdirectories → gstack-{dir}