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/manifesto/SKILL.md

@@ -0,0 +1,77 @@
+---
+name: manifesto
+description: >
+  View and manage the organization manifesto. Shows identity, personas, beliefs,
+  and principles. Use when viewing manifesto, checking organization identity, or
+  when user says /manifesto.
+model: haiku
+user-invocable: true
+---
+
+# Organization Manifesto
+
+@~/.claude/skills/vision-management/SKILL.md
+
+The manifesto defines the organization-level vision: who we are, who we serve, what we believe, and how we work. It is distinct from product-level vision (see `/vision`).
+
+## Process
+
+1. **Check for manifesto**: Look for `manifesto.md` in the current repo root.
+
+2. **If no manifesto exists**:
+   - Ask if the user wants to create one
+   - Guide through defining:
+     1. **Who We Are**: Organization identity
+     2. **Who We Serve**: 2-4 specific personas with context and constraints
+     3. **What They're Trying to Achieve**: Jobs to be done in their voice
+     4. **What We Believe**: Core beliefs including stance on AI-augmented development
+     5. **Guiding Principles**: Decision-making rules
+     6. **Non-Goals**: What we explicitly don't do
+   - Create `manifesto.md`
+
+3. **If manifesto exists**:
+   - Display formatted summary of the manifesto
+
+## Output Format
+
+When displaying an existing manifesto:
+
+```
+## Who We Are
+
+[Identity summary from manifesto]
+
+## Who We Serve
+
+- **[Persona 1]**: [Brief description]
+- **[Persona 2]**: [Brief description]
+- **[Persona 3]**: [Brief description]
+
+## What They're Trying to Achieve
+
+- "[Job to be done 1]"
+- "[Job to be done 2]"
+- "[Job to be done 3]"
+
+## What We Believe
+
+[Summary of key beliefs - especially AI-augmented development stance]
+
+## Guiding Principles
+
+1. [Principle 1]
+2. [Principle 2]
+3. [Principle 3]
+
+## Non-Goals
+
+- [Non-goal 1]
+- [Non-goal 2]
+```
+
+## Guidelines
+
+- The manifesto is the **organization-level** document - it applies across all products
+- Update rarely - this is foundational identity, not tactical direction
+- Product repos reference the manifesto but have their own `vision.md`
+- Use `/vision` for product-level vision management