fix: harden managed skill installs

[Damin-Lee]

Summary

• stop mutating source SKILL.md files when relinking Claude installs by rendering managed copies instead
• allow generated external-host output to live outside the source tree and update setup/relink tests accordingly
• fix Bun 1.3 node-server bundling by externalizing @ngrok/ngrok and make uninstall only report SessionStart hook removal when it actually removed one

Verification

• bun test
• bun run build

[Damin-Lee]

Mirrored this patch into my fork for local validation and manual merge flow: https://github.com/Damin-Lee/gstack/pull/1\n\nVerification on the patch branch:\n- bun test\n- bun run build\n\nOnce upstream review lands, I can switch the hac submodule pointer back to upstream main cleanly.