dx: Add PR overlap detection & alert (#12104)

## Summary

Adds an automated workflow that detects potential merge conflicts
between open PRs, helping contributors coordinate proactively.

**Example output:** [See comment on PR
#12057](https://github.com/Significant-Gravitas/AutoGPT/pull/12057#issuecomment-3897330632)

## How it works

1. **Triggered on PR events** — runs when a PR is opened, pushed to, or
reopened
2. **Compares against all open PRs** targeting the same base branch
3. **Detects overlaps** at multiple levels:
   - File overlap (same files modified)
   - Line overlap (same line ranges modified)
   - Actual merge conflicts (attempts real merges)
4. **Posts a comment** on the PR with findings

## Features

- Full file paths with common prefix extraction for readability
- Conflict size (number of conflict regions + lines affected)
- Conflict types (content, added, deleted, modified/deleted, etc.)
- Last-updated timestamps for each PR
- Risk categorization (conflict, medium, low)
- Ignores noise files (openapi.json, lock files)
- Updates existing comment on subsequent pushes (no spam)
- Filters out PRs older than 14 days
- Clone-once optimization for fast merge testing (~48s for 19 PRs)

## Files

- `.github/scripts/detect_overlaps.py` — main detection script
- `.github/workflows/pr-overlap-check.yml` — workflow definition

.github/workflows/pr-overlap-check.yml

@@ -0,0 +1,39 @@
+name: PR Overlap Detection
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+    branches:
+      - dev
+      - master
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  check-overlaps:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0  # Need full history for merge testing
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
+      - name: Configure git
+        run: |
+          git config user.email "github-actions[bot]@users.noreply.github.com"
+          git config user.name "github-actions[bot]"
+
+      - name: Run overlap detection
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        # Always succeed - this check informs contributors, it shouldn't block merging
+        continue-on-error: true
+        run: |
+          python .github/scripts/detect_overlaps.py ${{ github.event.pull_request.number }}