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/plan-issues/SKILL.md

@@ -0,0 +1,72 @@
+---
+name: plan-issues
+description: >
+  Plan and create issues for a feature or improvement. Breaks down work into
+  well-structured issues with vision alignment. Use when planning a feature,
+  creating a roadmap, breaking down large tasks, or when user says /plan-issues.
+model: sonnet
+argument-hint: <feature-description>
+context: fork
+user-invocable: true
+---
+
+# Plan Feature: $1
+
+@~/.claude/skills/gitea/SKILL.md
+@~/.claude/skills/roadmap-planning/SKILL.md
+@~/.claude/skills/issue-writing/SKILL.md
+@~/.claude/skills/vision-management/SKILL.md
+
+1. **Check vision context**: If `vision.md` exists, read it to understand personas, jobs to be done, and goals
+2. **Identify persona**: Which persona does "$1" serve?
+3. **Identify job**: Which job to be done does this enable?
+4. **Understand the feature**: Analyze what "$1" involves
+5. **Explore the codebase** if needed to understand context
+
+6. **Discovery phase**: Before proposing issues, walk through the user workflow:
+   - Who is the specific user?
+   - What is their goal?
+   - What is their step-by-step workflow to reach that goal?
+   - What exists today?
+   - Where does the workflow break or have gaps?
+   - What's the MVP that delivers value?
+
+   Present this as a workflow walkthrough before proposing any issues.
+
+7. **Break down** into discrete, actionable issues:
+   - Derive issues from the workflow gaps identified in discovery
+   - Each issue should be independently completable
+   - Clear dependencies between issues
+   - Appropriate scope (not too big, not too small)
+
+8. **Present the plan** (include vision alignment if vision exists):
+   ```
+   ## Proposed Issues for: $1
+
+   For: [Persona name]
+   Job: "[Job to be done this enables]"
+   Supports: [Milestone/Goal name]
+
+   1. [Title] - Brief description
+      Addresses gap: [which workflow gap this solves]
+      Dependencies: none
+
+   2. [Title] - Brief description
+      Addresses gap: [which workflow gap this solves]
+      Dependencies: #1
+
+   3. [Title] - Brief description
+      Addresses gap: [which workflow gap this solves]
+      Dependencies: #1, #2
+   ```
+
+   If the feature doesn't align with any persona/job/goal, note this and ask if:
+   - A new persona or job should be added to the vision
+   - A new milestone should be created
+   - This should be added as a non-goal
+   - Proceed anyway (with justification)
+
+9. **Ask for approval** before creating issues
+10. **Create issues** in dependency order (blockers first)
+11. **Link dependencies** using `tea issues deps add <issue> <blocker>` for each dependency
+12. **Present summary** with links to created issues and dependency graph