feat(skill): add /create-skill — skill factory with quality gates

[carlos-alm]

Summary

• Adds /create-skill skill that scaffolds, writes, and validates new Claude Code skills
• Encodes lessons from 200+ Greptile review comments across 4 titan PRs into 10 mandatory patterns and a 17-item self-review checklist
• Dogfooded against itself — caught and fixed 3 issues (hardcoded temp path in example, hardcoded lint command, missing exit conditions)

Motivation

The titan skill PRs accumulated 210 review comments (103 from Greptile alone) across 10 recurring issue categories. Most were structural consistency problems — shell vars across fences, stale step references, silent failures, internal contradictions. This skill prevents those issues upfront by making them part of the authoring process rather than catching them in review.

What the skill does

1. Phase 0 — Discovery: Gathers requirements interactively (purpose, args, phases, tools, artifacts, dangerous ops, resume support)
2. Phase 1 — Scaffold: Creates directory + SKILL.md with correct frontmatter and structure
3. Phase 2 — Write: Enforces 10 mandatory patterns derived from the top Greptile findings
4. Phase 3 — Guards: Adds safety guards for any dangerous operations
5. Phase 4 — Self-Review: 17-item checklist covering structure, anti-patterns, robustness, and completeness
6. Phase 5 — Finalize: User review + commit

Test plan

• Ran the skill's Phase 4 self-review checklist against itself
• Fixed 3 issues found during dogfooding
• Use it to create the next skill and verify it catches issues early