pr-implement

# PR Implement Skill

Fork-based implementation for open source contributions with mandatory isolation check.

## Overview

Execute a contribution plan with fork isolation. This skill ensures PRs are clean
and focused by running isolation checks before and during implementation.

**Input**: Plan artifact from `/pr-plan` or repo URL

**When to Use**:
- Implementing a planned OSS contribution
- Need isolation enforcement for clean PRs
- Working in a fork worktree
- After completing `/pr-plan`

**When NOT to Use**:
- Internal project work (use `/implement`)
- Haven't planned yet (run `/pr-plan` first)
- Trivial changes (implement directly)

---

## Workflow

```
-1. Prior Work Check      -> BLOCKING: Check for competing PRs before starting
0.  Input Discovery       -> Find plan artifact or repo
1.  Fork Setup            -> Ensure fork exists and is current
2.  Worktree Creation     -> Create isolated worktree for changes
3.  Isolation Pre-Check   -> BLOCK if existing branch has mixed concerns
4.  Implementation        -> Execute plan with progress tracking
5.  Isolation Post-Check  -> BLOCK if implementation mixed concerns
6.  Commit Preparation    -> Stage changes with proper commit type
7.  Handoff               -> Ready for /pr-prep
```

---

## Phase -1: Prior Work Check (BLOCKING)

**CRITICAL**: Before starting implementation, verify no one else is working on this.

### -1.1 Check for Competing PRs

```bash
# Get target repo from plan or context
REPO="<owner/repo>"
TOPIC="<topic from plan>"

# Search for open PRs on this topic
gh pr list -R $REPO --state open --search "$TOPIC" --limit 10

# Check for PRs touching same files
gh pr list -R $REPO --state open --json files,number,title | \
  jq -r '.[] | select(.files[].path | contains("<target-file>")) | "#\(.number): \(.title)"'
```

### -1.2 Check Target Issue Status

```bash
# If implementing a specific issue, verify it's still available
gh issue view <issue-number> -R $REPO --json state,assignees,comments

# Chec