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>

scripts/resolvers/codex-helpers.ts

@@ -61,7 +61,8 @@ policy:
 `;
 }
 
-export function codexSkillName(skillDir: string): string {
+/** Compute skill name for external hosts (Codex, Factory, etc.) */
+export function externalSkillName(skillDir: string): string {
   if (skillDir === '.' || skillDir === '') return 'gstack';
   // Don't double-prefix: gstack-upgrade → gstack-upgrade (not gstack-gstack-upgrade)
   if (skillDir.startsWith('gstack-')) return skillDir;