feat(issue): add /issue skill for backlog-ready GitHub issue authoring

Interrogates an ambiguous request through five strict phases (why, scope,
technical, draft, final) and produces a GitHub issue precise enough that an
unfamiliar engineer or AI agent can execute it without follow-up. Slots in
after /office-hours (when the idea has passed the "worth building" bar) and
before /plan-eng-review (which assumes a plan already exists).

- issue/SKILL.md.tmpl + generated SKILL.md
- routing entry in root SKILL.md.tmpl
- llms.txt regenerated to include the new skill

SKILL.md

@@ -486,6 +486,7 @@ quality gates that produce better results than answering inline.
 
 **Routing rules — when you see these patterns, INVOKE the skill via the Skill tool:**
 - User describes a new idea, asks "is this worth building", brainstorms, pitches a concept → invoke `/office-hours`
+- User asks to file an issue, write up a ticket, "turn this into a GitHub issue", "spec this out" → invoke `/issue`
 - User asks about strategy, scope, ambition, "think bigger", "what should we build" → invoke `/plan-ceo-review`
 - User asks to review architecture, lock in the plan, "does this design make sense" → invoke `/plan-eng-review`
 - User asks about design system, brand, visual identity, "how should this look" → invoke `/design-consultation`