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/examples/simple-workflow.md

@@ -0,0 +1,71 @@
+# Example: Simple Workflow Skill
+
+A basic skill with just a SKILL.md file - no scripts or reference files needed.
+
+## Structure
+
+```
+skills/list-open-prs/
+└── SKILL.md
+```
+
+## When to Use
+
+- Skill is simple (<300 lines)
+- No error-prone bash operations
+- No need for reference documentation
+- Straightforward workflow
+
+## Example: list-open-prs
+
+```markdown
+---
+name: list-open-prs
+description: >
+  List all open pull requests for the current repository.
+  Use when user wants to see PRs or says /list-open-prs.
+model: haiku
+user-invocable: true
+---
+
+# List Open PRs
+
+@~/.claude/skills/gitea/SKILL.md
+
+Show all open pull requests in the current repository.
+
+## Process
+
+1. **Get repository info**
+   - `git remote get-url origin`
+   - Parse owner/repo from URL
+
+2. **Fetch open PRs**
+   - `tea pulls list --state open --output simple`
+
+3. **Format results** as table
+
+| PR # | Title | Author | Created |
+|------|-------|--------|---------|
+| ... | ... | ... | ... |
+
+## Guidelines
+
+- Show most recent PRs first
+- Include link to each PR
+- If no open PRs, say "No open pull requests"
+```
+
+## Why This Works
+
+- **Concise**: Entire skill fits in ~30 lines
+- **Simple commands**: Just git and tea CLI
+- **No error handling needed**: tea handles errors gracefully
+- **Structured output**: Table format is clear
+
+## What Makes It Haiku-Friendly
+
+- ✓ Simple sequential steps
+- ✓ Clear commands with no ambiguity
+- ✓ Structured output format
+- ✓ No complex decision-making