feat(skills): modernize capability-writing with Anthropic best practices

Updates capability-writing skill with progressive disclosure structure based on
Anthropic's January 2025 documentation. Implements Haiku-first approach (12x
cheaper, 2-5x faster than Sonnet).

Key changes:
- Add 5 core principles: conciseness, progressive disclosure, script bundling,
  degrees of freedom, and Haiku-first model selection
- Restructure with best-practices.md, templates/, examples/, and reference/
- Create 4 templates: user-invocable skill, background skill, agent, helper script
- Add 3 examples: simple workflow, progressive disclosure, with scripts
- Add 3 reference docs: frontmatter fields, model selection, anti-patterns
- Update create-capability to analyze complexity and recommend structures
- Default all new skills/agents to Haiku unless justified

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

skills/capability-writing/templates/user-invocable-skill.md

@@ -0,0 +1,65 @@
+---
+name: skill-name
+description: >
+  Clear description of what this skill does and when to use it.
+  Use when [specific trigger conditions] or when user says /skill-name.
+model: haiku
+argument-hint: <required-param> [optional-param]
+user-invocable: true
+# context: fork  # Use for skills needing isolated context
+# allowed-tools:  # Restrict tools if needed
+#   - Read
+#   - Bash
+---
+
+# Skill Title
+
+@~/.claude/skills/relevant-background-skill/SKILL.md
+
+Brief intro explaining the skill's purpose (1-2 sentences max).
+
+## Process
+
+1. **First step**: Clear action with specific command or instruction
+   - `command or tool to use`
+   - What to look for or validate
+
+2. **Second step**: Next action
+   - Specific details
+   - Expected output
+
+3. **Ask for approval** before significant actions
+   - Show what will be created/modified
+   - Wait for user confirmation
+
+4. **Execute** the approved actions
+   - Run commands/create files
+   - Handle errors gracefully
+
+5. **Present results** with links and summary
+   - Structured output (table or list)
+   - Links to created resources
+
+## Guidelines
+
+- Keep responses concise
+- Use structured output (tables, lists)
+- No preambles or sign-offs
+- Validate inputs before acting
+
+## Output Format
+
+Use this structure for responses:
+
+\`\`\`
+## Summary
+[1-2 sentences]
+
+## Results
+| Item | Status | Link |
+|------|--------|------|
+| ... | ... | ... |
+
+## Next Steps
+- ...
+\`\`\`