fix: namespace generated Hermes skill names

[dkoh12]

Summary

• add host-config support for host-specific emitted skill names
• namespace generated Hermes skill frontmatter names to match their gstack-* directories
• add a regression test covering Hermes skill-name generation

Problem

Hermes deduplicates skills by frontmatter name, not by directory name. Generated Hermes skills were written to namespaced directories like gstack-review/, but their frontmatter still used generic names like review, causing Hermes to suppress them as duplicates of existing skills.

Test Plan

• bun test test/gen-skill-docs.test.ts -t "Hermes frontmatter names are namespaced to match generated skill names"
• bun run scripts/gen-skill-docs.ts --host hermes
• verify hermes skills list shows gstack-review, gstack-qa, and gstack-ship