refactor: migrate commands to user-invocable skills

Claude Code has unified commands into skills with the user-invocable
frontmatter field. This migration:

- Converts 20 commands to skills with user-invocable: true
- Consolidates docs into single writing-capabilities.md
- Rewrites capability-writing skill for unified model
- Updates CLAUDE.md, Makefile, and other references
- Removes commands/ directory

Skills now have two types:
- user-invocable: true - workflows users trigger with /name
- user-invocable: false - background knowledge auto-loaded

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

skills/create-capability/SKILL.md

@@ -0,0 +1,117 @@
+---
+name: create-capability
+description: >
+  Create a new capability (skill, agent, or a cohesive set) for the architecture
+  repository. Use when creating new skills, agents, extending AI workflows, or when
+  user says /create-capability.
+model: haiku
+argument-hint: <description>
+user-invocable: true
+---
+
+# Create Capability
+
+@~/.claude/skills/capability-writing/SKILL.md
+
+Create new capabilities following established patterns. A capability may be a single component or a cohesive set (skill + agent).
+
+## Process
+
+1. **Understand the capability**: Analyze "$1" to understand what the user wants to build
+   - What domain or workflow does this cover?
+   - What user need does it address?
+   - What existing capabilities might overlap?
+
+2. **Determine components needed**: Based on the description, recommend which components:
+
+   | Pattern | When to Use |
+   |---------|-------------|
+   | Skill only (background) | Knowledge to apply automatically (reused across other skills) |
+   | Skill only (user-invocable) | User-invoked workflow |
+   | Skill + Agent | Workflow with isolated worker for complex subtasks |
+   | Full set | New domain expertise + workflow + isolated work |
+
+   Present recommendation with reasoning:
+   ```
+   ## Recommended Components for: $1
+
+   Based on your description, I recommend:
+   - **Skill**: `name` - [why this knowledge is needed]
+   - **Agent**: `name` - [why isolation/specialization is needed] (optional)
+
+   Reasoning: [explain why this combination fits the need]
+   ```
+
+3. **Gather information**: For each recommended component, ask:
+
+   **For all components:**
+   - Name (kebab-case, descriptive)
+   - Description (one-line summary)
+
+   **For Skills:**
+   - What domain/knowledge does this cover?
+   - What are the key concepts to teach?
+   - What patterns or templates should it include?
+   - Is it user-invocable (workflow) or background (reference)?
+
+   **For Agents:**
+   - What specialized role does this fill?
+   - What skills does it need?
+   - Should it be read-only (no Edit/Write)?
+
+4. **Select appropriate models**:
+
+   | Model | Use For |
+   |-------|---------|
+   | `haiku` | Simple fetch/display skills, formatting tasks |
+   | `sonnet` | Most skills and agents (default) |
+   | `opus` | Deep reasoning, architectural analysis, complex judgment |
+
+   For each component, recommend a model with reasoning.
+
+5. **Generate files**: Create content using templates from capability-writing skill
+
+   Ensure proper inter-references:
+   - User-invocable skill references background skills via `@~/.claude/skills/name/SKILL.md`
+   - Agent lists skills in `skills:` frontmatter (names only, not paths)
+   - User-invocable skill spawns agent via Task tool if agent is part of the set
+
+6. **Present for approval**: Show all generated files with their full content:
+   ```
+   ## Generated Files
+
+   ### skills/name/SKILL.md
+   [full content]
+
+   ### agents/name/AGENT.md (if applicable)
+   [full content]
+
+   Ready to create these files?
+   ```
+
+7. **Create files** in correct locations after approval:
+   - `skills/<name>/SKILL.md`
+   - `agents/<name>/AGENT.md`
+
+8. **Report success**:
+   ```
+   ## Capability Created: name
+
+   Files created:
+   - skills/name/SKILL.md
+   - agents/name/AGENT.md (if applicable)
+
+   Next steps:
+   1. Run `make install` to symlink to ~/.claude/
+   2. Test with: /name (for user-invocable skills)
+   3. Background skills will auto-activate based on context
+   ```
+
+## Guidelines
+
+- Follow all conventions from capability-writing skill
+- Reference existing skills rather than duplicating knowledge
+- Keep components focused - split if scope is too broad
+- User-invocable skills should have approval checkpoints before significant actions
+- Default to `sonnet` model unless there's a clear reason for haiku/opus
+- Skills should have descriptive `description` fields for auto-activation