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/spawn-pr-fixes/SKILL.md

@@ -0,0 +1,124 @@
+---
+name: spawn-pr-fixes
+description: Spawn parallel background agents to address PR review feedback
+model: haiku
+argument-hint: [pr-number...]
+allowed-tools: Bash, Task, Read
+user-invocable: true
+---
+
+# Spawn PR Fixes
+
+Spawn background agents to address review feedback on multiple PRs in parallel. Each agent works in an isolated git worktree.
+
+## Arguments
+
+Optional PR numbers separated by spaces: `$ARGUMENTS`
+
+- With arguments: `/spawn-pr-fixes 12 15 18` - fix specific PRs
+- Without arguments: `/spawn-pr-fixes` - find and fix all PRs with requested changes
+
+## Process
+
+### Step 1: Get Repository Info
+
+```bash
+REPO_PATH=$(pwd)
+REPO_NAME=$(basename $REPO_PATH)
+```
+
+### Step 2: Determine PRs to Fix
+
+**If PR numbers provided**: Use those directly
+
+**If no arguments**: Find PRs needing work
+```bash
+# List open PRs
+tea pulls --state open
+
+# For each PR, check if it has review comments requesting changes
+tea pulls <number> --comments
+```
+
+Look for PRs where:
+- Review comments exist that haven't been addressed
+- PR is not approved yet
+- PR is open (not merged/closed)
+
+### Step 3: For Each PR
+
+1. Fetch PR title using `tea pulls <number>`
+
+2. Spawn background agent using Task tool:
+   ```
+   Task tool with:
+     - subagent_type: "pr-fixer"
+     - run_in_background: true
+     - prompt: See agent prompt below
+   ```
+
+### Agent Prompt
+
+For each PR, use this prompt:
+
+```
+You are a pr-fixer agent. Address review feedback on PR #<NUMBER> autonomously.
+
+Context:
+- Repository path: <REPO_PATH>
+- Repository name: <REPO_NAME>
+- PR number: <NUMBER>
+
+Instructions from @agents/pr-fixer/agent.md:
+
+1. Get PR details and review comments:
+   cd <REPO_PATH>
+   git fetch origin
+   tea pulls <NUMBER> --comments
+
+2. Setup worktree from PR branch:
+   git worktree add ../<REPO_NAME>-pr-<NUMBER> origin/<branch-name>
+   cd ../<REPO_NAME>-pr-<NUMBER>
+   git checkout <branch-name>
+
+3. Analyze feedback, create todos with TodoWrite
+
+4. Address each piece of feedback
+
+5. Commit and push:
+   git add -A && git commit with message "Address review feedback\n\n...\n\nCo-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>"
+   git push
+
+6. Spawn code-reviewer synchronously (NOT in background) to re-review
+
+7. If needs more work, fix and re-review (max 3 iterations)
+
+8. Cleanup (ALWAYS do this):
+   cd <REPO_PATH> && git worktree remove ../<REPO_NAME>-pr-<NUMBER> --force
+
+9. Output concise summary (5-10 lines max):
+   PR #<NUMBER>: <title>
+   Status: <fixed|partial|blocked>
+   Feedback addressed: <count> items
+   Review: <approved|needs-work|skipped>
+
+Work autonomously. Make judgment calls on ambiguous feedback. If blocked, note it in a commit message.
+```
+
+### Step 4: Report
+
+After spawning all agents, display:
+
+```
+Spawned <N> pr-fixer agents:
+
+| PR  | Title                    | Status     |
+|-----|--------------------------|------------|
+| #12 | Add /commit command      | spawned    |
+| #15 | Add /pr command          | spawned    |
+| #18 | Add CI status            | spawned    |
+
+Agents working in background. Monitor with:
+- Check PR list: tea pulls
+- Check worktrees: git worktree list
+```