feat: add feature-scope skill

[malikrohail]

Summary

Adds /feature-scope, a new planning-stage skill that sits between ideation and implementation.

What it does

• Turns a promising idea into a concrete v1 scope
• Defines in-scope vs out-of-scope explicitly
• Writes acceptance criteria and blocking questions
• Breaks work into independently shippable implementation slices
• Saves a scoped feature doc under ~/.gstack/projects/
• Hands off to /plan-eng-review or /plan-ceo-review depending on whether the remaining ambiguity is technical or strategic

Repo integration

• New skill template and generated skill:
  • feature-scope/SKILL.md.tmpl
  • feature-scope/SKILL.md
• Added to root skill routing text, README, AGENTS, CLAUDE repo map, and docs/skills deep dive
• Added to skill:check
• Added structural validation coverage in test/skill-validation.test.ts
• Generated Codex host variant as gstack-feature-scope

Why this fills a real gap

/office-hours gives you the right problem framing, but there is still a missing step before architecture review and implementation: defining the smallest valuable version, what is explicitly deferred, and how the work should be sliced.

This PR makes that intermediate scoping step first-class.

Verification

• bun scripts/gen-skill-docs.ts
• bun run scripts/gen-skill-docs.ts --host codex
• bun run skill:check
• bun test test/skill-validation.test.ts -t "feature-scope skill structure"
• bun test

Refs #318