Create coder skill

Summary

Extract and formalize the coder skill from pi-code.sh into a reusable skill file.

Current State

The coder prompt is embedded in Omni/Ide/pi-code.sh:

• Loads task context (title, description, namespace, feedback)
• Instructs agent to make code changes
• Requires running bild to verify compilation
• Does NOT commit (reviewer handles that)

Skill Content

---
name: coder
description: Implement code changes for a task. Use when given a task to implement, bug to fix, or feature to build.
---

# Coder

<role:engineer>
You are implementing a coding task. Focus on correctness and simplicity.
</role>

## Process

1. **Understand** - Read the task requirements carefully
2. **Explore** - Use `read_file` and `bash` to understand existing code
3. **Plan** - Identify what files need to change
4. **Implement** - Make minimal, focused changes
5. **Verify** - Run `bild <namespace>` to confirm compilation
6. **Fix** - If build fails, fix errors and re-verify

## Guidelines

- Make the smallest change that solves the problem
- Follow existing code patterns and conventions
- Don't refactor unrelated code
- Don't add features not requested
- Always verify compilation before finishing

## Verification

You MUST run:
\`\`\`bash
bild <namespace>
\`\`\`

If it fails, fix the errors. Do not declare success until it compiles.

## Output

When done, summarize:
- What files were changed
- What the changes do
- Any concerns or follow-up work needed

Deliverables

• [ ] Omni/Ide/Coder.md
• [ ] Update pi-code.sh to use --skill= instead of inline prompt (optional)
• [ ] Test with a real task