feat: add /feature-scope skill — guided scoping before implementation

[jnMetaCode]

Summary

Adds /feature-scope — a new skill that bridges the gap between /office-hours (idea validation) and /plan-eng-review (architecture lockdown). This is the missing step identified in #318.

Lifecycle position:

/office-hours → /feature-scope → /plan-eng-review → implement → /review → /ship

What it does

Five phases:

Phase	What	Output
1. Context Gathering	Reads design docs, TODOS, git history, codebase	Project understanding
2. Goal Clarification	5 forcing questions via AskUserQuestion	Clear requirements
3. Scope Definition	Acceptance criteria, deferred items, risks	Scope doc draft
4. Implementation Slices	Ordered, independently shippable PRs (<500 LOC each)	Implementation plan
5. Save & Handoff	Persists to ~/.gstack/projects/$SLUG/	Saved doc + next steps

Hard gate: Produces a scoping doc, not code. Same pattern as /office-hours.

The 5 forcing questions

1. Who is the user? — Push for specificity ("a backend engineer at 2am" not "developers")
2. What does "done" look like? — Concrete acceptance moment
3. What's the simplest valuable version? — Strip to MVP
4. What's explicitly NOT in v1? — Minimum 3 deferred items (prevents scope creep)
5. What existing code does this touch? — Risk assessment from codebase scan

Operating principles

• Scope is a decision, not a discovery
• Deferred is not deleted
• Acceptance criteria are tests
• Small slices ship faster

Files

File	Purpose
feature-scope/SKILL.md.tmpl	Template source (uses {{PREAMBLE}}, {{SLUG_EVAL}})
feature-scope/SKILL.md	Generated output (needs bun run gen:skill-docs after merge to inject standard preamble)

Test plan

• /feature-scope produces a scoping doc with all 6 sections
• Forcing questions use AskUserQuestion (interactive)
• Reads existing /office-hours design docs when available
• Saves output to ~/.gstack/projects/$SLUG/feature-scope-*.md
• Suggests /plan-eng-review or /plan-ceo-review as next steps
• Does NOT write any implementation code (hard gate)

Closes #318