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/review-pr/SKILL.md

@@ -0,0 +1,90 @@
+---
+name: review-pr
+description: >
+  Review a Gitea pull request. Fetches PR details, diff, and comments. Includes
+  both code review and software architecture review. Use when reviewing pull requests,
+  checking code quality, or when user says /review-pr.
+model: sonnet
+argument-hint: <pr-number>
+user-invocable: true
+---
+
+# Review PR #$1
+
+@~/.claude/skills/gitea/SKILL.md
+@~/.claude/skills/software-architecture/SKILL.md
+
+## 1. Gather Information
+
+1. **View PR details** with `--comments` flag to see description, metadata, and discussion
+2. **Get the diff** to review the changes:
+   ```bash
+   tea pulls checkout <number>
+   git diff main...HEAD
+   ```
+
+## 2. Code Review
+
+Review the changes and provide feedback on:
+- Code quality and style
+- Potential bugs or logic errors
+- Test coverage
+- Documentation updates
+
+## 3. Software Architecture Review
+
+Spawn the software-architect agent for architectural analysis:
+
+```
+Task tool with:
+  - subagent_type: "software-architect"
+  - prompt: |
+      ANALYSIS_TYPE: pr-review
+      TARGET: <pr-number>
+      CONTEXT: [Include the PR diff and description]
+```
+
+The architecture review checks:
+- **Pattern consistency**: Changes follow existing codebase patterns
+- **Dependency direction**: Dependencies flow correctly (toward domain layer)
+- **Breaking changes**: API changes are flagged and justified
+- **Module boundaries**: Changes respect existing package boundaries
+- **Error handling**: Errors wrapped with context, proper error types used
+
+## 4. Present Findings
+
+Structure the review with two sections:
+
+### Code Review
+- Quality, bugs, style issues
+- Test coverage gaps
+- Documentation needs
+
+### Architecture Review
+- Summary of architectural concerns from agent
+- Pattern violations or anti-patterns detected
+- Dependency or boundary issues
+- Breaking change assessment
+
+## 5. User Actions
+
+Ask the user what action to take:
+- **Merge**: Post review summary as comment, then merge with rebase style
+- **Request changes**: Leave feedback without merging
+- **Comment only**: Add a comment for discussion
+
+## Merging
+
+Always use tea CLI for merges to preserve user attribution:
+
+```bash
+tea pulls merge <number> --style rebase
+```
+
+For review comments, use `tea comment` since `tea pulls review` is interactive-only:
+
+```bash
+tea comment <number> "<review summary>"
+```
+
+> **Warning**: Never use the Gitea API with admin credentials for user-facing operations like merging. This causes the merge to be attributed to the admin account instead of the user.