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/improve/SKILL.md

@@ -0,0 +1,89 @@
+---
+name: improve
+description: >
+  Identify improvement opportunities based on product vision. Analyzes gaps between
+  vision goals and current backlog. Use when analyzing alignment, finding gaps, or
+  when user says /improve.
+model: sonnet
+context: fork
+user-invocable: true
+---
+
+# Improvement Analysis
+
+@~/.claude/skills/vision-management/SKILL.md
+@~/.claude/skills/gitea/SKILL.md
+@~/.claude/skills/issue-writing/SKILL.md
+@~/.claude/skills/roadmap-planning/SKILL.md
+
+## Process
+
+1. **Read the vision**: Load `vision.md` from the repo root.
+   - If no vision exists, suggest running `/vision` first
+
+2. **Fetch current backlog**: Get all open issues from Gitea using `tea issues`
+
+3. **Analyze alignment**:
+
+   For each vision goal, check:
+   - Are there issues supporting this goal?
+   - Is there recent activity/progress?
+   - Are issues blocked or stalled?
+
+   For each open issue, check:
+   - Does it align with a vision goal?
+   - Is it supporting the current focus?
+
+4. **Identify gaps and opportunities**:
+
+   - **Unsupported goals**: Vision goals with no issues
+   - **Stalled goals**: Goals with issues but no recent progress
+   - **Orphan issues**: Issues that don't support any goal
+   - **Focus misalignment**: Issues not aligned with current focus getting priority
+   - **Missing non-goals**: Patterns suggesting things we should explicitly avoid
+
+5. **Present findings**:
+
+   ```
+   ## Vision Alignment Report
+
+   ### Goals Coverage
+   - Goal 1: [status] - N issues, [progress]
+   - Goal 2: [status] - N issues, [progress]
+
+   ### Gaps Identified
+   1. [Gap description]
+      Suggestion: [concrete action]
+
+   2. [Gap description]
+      Suggestion: [concrete action]
+
+   ### Orphan Issues
+   - #N: [title] - No goal alignment
+
+   ### Recommended Actions
+   1. [Action with rationale]
+   2. [Action with rationale]
+   ```
+
+6. **Offer to take action**:
+
+   For unsupported goals:
+   - Ask if user wants to plan issues for the gap
+   - If yes, run the `/plan-issues` workflow for that goal
+   - This breaks down the goal into concrete, actionable issues
+
+   For other findings:
+   - Re-prioritize issues based on focus
+   - Close or re-scope orphan issues
+   - Update vision with suggested changes
+
+   Always ask for approval before making changes.
+
+## Guidelines
+
+- Focus on actionable improvements, not just observations
+- Prioritize suggestions by impact on vision goals
+- Keep suggestions specific and concrete
+- One issue per improvement (don't bundle)
+- Reference specific goals when suggesting new issues