feat: add meta-skill create-skill for creating and improving skills

Two modes: Create (gather requirements, generate SKILL.md) and Improve
(diagnose existing skill against best practices, propose changes).
Includes bundled references for frontmatter spec and writing guide.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude/skills/create-skill/SKILL.md

@@ -0,0 +1,119 @@
+---
+name: create-skill
+description: Create or improve a Claude Code skill — gathers requirements, selects agent type and frontmatter, generates a SKILL.md following best practices. Use when the user wants to create a skill, turn a workflow into a skill, improve an existing skill, or automate a repeating task as a slash command.
+disable-model-invocation: true
+argument-hint: "[skill-name or 'improve existing-skill-name']"
+---
+
+# Create or Improve Skill
+
+Target: `$ARGUMENTS`
+
+## Context
+
+Existing project skills:
+!`ls .claude/skills/ 2>/dev/null`
+
+## Detect Mode
+
+- If `$ARGUMENTS` names an existing skill (check dirs above) → **Improve mode**
+- Otherwise → **Create mode**
+
+---
+
+## Create Mode
+
+### 1. Capture intent
+
+Check if the current conversation already contains a workflow the user wants to turn into a skill. If so, extract: tools used, sequence of steps, corrections made, input/output formats. Confirm with the user before proceeding.
+
+If no prior context, gather requirements:
+- What does this skill do? (one sentence)
+- When should someone use it? (trigger phrases, contexts)
+- What arguments does it expect? (file path, name, or none)
+- Does it produce new files, modify existing files, or output analysis?
+- Does it need live data at invocation time? (git diff, shell output)
+- Scope: personal skill (all projects) or project-only?
+
+### 2. Research
+
+Read 2-3 existing skills closest in nature to the user's intent. Use them as examples of project conventions.
+
+Read `${CLAUDE_SKILL_DIR}/references/frontmatter-spec.md` for the complete frontmatter specification.
+Read `${CLAUDE_SKILL_DIR}/references/writing-guide.md` for skill writing best practices.
+
+### 3. Determine frontmatter
+
+Select the appropriate `agent` based on domain:
+
+| Agent | Domain |
+|---|---|
+| `frontend-architect` | UI, components, accessibility |
+| `backend-architect` | APIs, databases, server logic |
+| `code-reviewer` | Code review, quality analysis |
+| `test-engineer` | Tests, test strategy |
+| `security-auditor` | Security, threat modeling |
+| `prompt-engineer` | Prompts, LLM config |
+| `documentation-expert` | Docs, ADRs |
+| _(omit + no fork)_ | Multi-file coordination, interactive workflows |
+
+Decide: `context: fork` for standalone deliverables (new file, analysis report). Omit for interactive workflows or multi-file coordination.
+
+Always include `disable-model-invocation: true` (project convention).
+
+### 4. Write the SKILL.md
+
+Structure:
+- H1 title — short action phrase
+- One-liner referencing `$ARGUMENTS`
+- Context section (optional) — `!` backtick for live data
+- Steps — numbered, imperative:
+  - Step 1: read project context
+  - Middle: core logic
+  - Last: output or summary
+- Output format (if structured) — markdown template in fenced block
+
+### 5. Write file
+
+Based on scope choice:
+- Personal: `~/.claude/skills/$NAME/SKILL.md`
+- Project: `.claude/skills/$NAME/SKILL.md`
+
+### 6. Verify and suggest tests
+
+Read back the file and check:
+- [ ] Valid YAML frontmatter
+- [ ] Lowercase-hyphen name, no collision with existing skills
+- [ ] Description with trigger contexts (slightly "pushy")
+- [ ] Body: H1, steps, $ARGUMENTS reference
+- [ ] Length appropriate (30-70 focused, up to 500 complex)
+
+Propose 2-3 realistic test prompts including one edge case.
+
+---
+
+## Improve Mode
+
+### 1. Read the existing skill
+
+Read the full SKILL.md and any bundled resources in its directory.
+
+### 2. Diagnose issues
+
+Read `${CLAUDE_SKILL_DIR}/references/writing-guide.md` for evaluation criteria.
+
+Analyze against:
+- Description quality — is it "pushy" enough? Does it cover trigger contexts?
+- Instruction clarity — does it explain WHY, not just rules?
+- Structure — follows project conventions?
+- Length — is it concise or bloated?
+- Filler — does every sentence change agent behavior?
+
+### 3. Propose improvements
+
+Show a before/after diff of proposed changes with rationale for each change.
+Apply only after user approval.
+
+### 4. Suggest test prompts
+
+Propose 2-3 prompts to verify the improved skill works as expected.