[codex] default Claude installs to namespaced commands

[malikrohail]

Summary

• make namespaced /gstack-* commands the default for fresh Claude installs
• update setup prompt copy to explain that namespaced commands avoid host-command collisions like /autoplan vs Claude Plan mode
• document the /gstack-autoplan fallback in README troubleshooting and installation guidance

Root Cause

Fresh Claude installs defaulted to flat slash commands like /autoplan, /review, and /qa whenever no saved skill_prefix preference existed. That made gstack more vulnerable to host slash-command collisions before the skill ever ran.

Validation

• bun test test/gen-skill-docs.test.ts
• bun test test/relink.test.ts
• bun run skill:check

Fixes #690.