chore(spec): rename /issue → /spec + fix duplicate analytics block

Foundation commit for the /spec skill (extends PR #1698 by @jayzalowitz).

- Renames issue/ → spec/ (template + generated)
- Removes the hand-rolled analytics block in spec/SKILL.md.tmpl (lines 46-49 of the original); {{PREAMBLE}} already emits the analytics write with the telemetry opt-out guard, so the duplicate would have bypassed gstack-config set telemetry off
- Updates frontmatter (name: spec, expanded description with magical-moment preview, triggers reordered to lead with "spec this out")
- Updates root SKILL.md.tmpl routing entry → /spec
- Regenerates spec/SKILL.md and gstack/llms.txt via bun run gen:skill-docs

Co-Authored-By: Jay Zalowitz <jayzalowitz@gmail.com>
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

SKILL.md

@@ -486,7 +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 to spec something out, file an issue, write up a ticket, "turn this into a GitHub issue", "backlog item" → invoke `/spec`
 - 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`