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-issue/SKILL.md

@@ -0,0 +1,48 @@
+---
+name: create-issue
+description: >
+  Create a new Gitea issue. Can create single issues or batch create from a plan.
+  Use when creating issues, adding tickets, or when user says /create-issue.
+model: haiku
+argument-hint: [title] or "batch"
+user-invocable: true
+---
+
+# Create Issue(s)
+
+@~/.claude/skills/gitea/SKILL.md
+
+## Milestone Assignment
+
+Before creating issues, fetch available milestones:
+
+```bash
+tea milestones -f title,description
+```
+
+For each issue, automatically assign to the most relevant milestone by matching:
+- Issue content/problem area → Milestone title and description
+- If no clear match, ask the user which milestone (goal) the issue supports
+- If no milestones exist, skip milestone assignment
+
+Include `--milestone "<milestone>"` in the create command when a milestone is assigned.
+
+## Single Issue (default)
+
+If title provided:
+1. Create an issue with that title
+2. Ask for description
+3. Assign to appropriate milestone (see above)
+4. Ask if this issue depends on any existing issues
+5. If dependencies exist, link them: `tea issues deps add <new-issue> <blocker>`
+
+## Batch Mode
+
+If $1 is "batch":
+1. Ask user for the plan/direction
+2. Fetch available milestones
+3. Generate list of issues with titles, descriptions, milestone assignments, and dependencies
+4. Show for approval
+5. Create each issue with milestone (in dependency order)
+6. Link dependencies between created issues: `tea issues deps add <issue> <blocker>`
+7. Display all created issue numbers with dependency graph