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/references/frontmatter-spec.md

@@ -0,0 +1,83 @@
+# Frontmatter Specification
+
+Complete reference for SKILL.md YAML frontmatter fields.
+
+## Required Fields
+
+### `name`
+- Lowercase-hyphen format: `my-skill-name`
+- Max 64 characters
+- Must be unique across all skill directories
+- Becomes the slash command: `/my-skill-name`
+
+### `description`
+- One sentence, under 200 characters
+- Should be slightly "pushy" — describe WHEN to use the skill, not just what it does
+- Include trigger contexts so the model selects the skill proactively
+- Good: "Scaffold a new React component with TypeScript, accessibility, all states, and variant support via cva."
+- Bad: "Creates components."
+- Pattern: `[Action verb] [what] — [key details]. Use when [trigger contexts].`
+
+## Optional Fields
+
+### `disable-model-invocation`
+- `true` — skill only runs when user invokes via `/name`
+- `false` (default) — model can auto-invoke when description matches user intent
+- Project convention: always set to `true`
+
+### `user-invocable`
+- `true` (default) — appears as a slash command
+- `false` — background knowledge only, used as reference by other skills
+
+### `argument-hint`
+- Shown in autocomplete: `/skill-name [hint]`
+- Use brackets for placeholders: `"[file-or-directory]"`, `"[ComponentName]"`
+- Omit if the skill takes no arguments
+
+### `context`
+- `fork` — runs in a forked context (separate conversation branch)
+  - Use for: standalone deliverables, analysis reports, code generation that doesn't need follow-up
+  - The skill gets its own clean context with only the skill instructions
+- Omit (no fork) — runs in the main conversation context
+  - Use for: interactive workflows, multi-step conversations, skills that need conversation history
+  - When omitted, `agent` field should also be omitted
+
+Decision tree:
+1. Does the skill need conversation history? → no fork
+2. Does the skill ask the user questions mid-flow? → no fork
+3. Does the skill produce a single standalone output? → fork
+4. Is it a one-shot analysis or generation? → fork
+
+### `agent`
+- Assigns a specialized agent persona. Only meaningful with `context: fork`.
+- Available agents:
+
+| Agent | Use for |
+|---|---|
+| `frontend-architect` | UI, components, CSS, accessibility, browser APIs |
+| `backend-architect` | APIs, databases, server logic, system design |
+| `code-reviewer` | Code review, quality analysis, diff review |
+| `test-engineer` | Test writing, test strategy, coverage analysis |
+| `security-auditor` | Security audit, threat modeling, vulnerability analysis |
+| `prompt-engineer` | Prompt design, LLM configuration, prompt evaluation |
+| `documentation-expert` | Documentation, ADRs, technical writing |
+
+- Omit when: no fork, or skill doesn't fit a specific domain
+
+### `allowed-tools`
+- Restrict which tools the skill can use
+- Comma-separated list: `allowed-tools: Read, Grep, Glob`
+- Omit to allow all tools (default)
+
+### `model`
+- Override the model for this skill
+- Example: `model: claude-sonnet-4-6`
+- Omit to use the default model
+
+### `hooks`
+- Lifecycle event handlers (pre/post execution)
+- Rarely needed for most skills
+
+### `compatibility`
+- Declare dependencies or requirements
+- Example: `compatibility: node>=18`