Add soft-gate PR risk scanning automation

Introduce a PR risk scanner script plus two workflows: one to scan changed files and upload findings, and one to upsert a sticky PR comment with a summary table and findings. This adds non-blocking supply-chain risk visibility for agentic contributions.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/pr-risk-scan-comment.yml

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+name: PR Risk Scan — Comment
+
+on:
+  workflow_run:
+    workflows: ["PR Risk Scan — Gate"]
+    types: [completed]
+
+permissions:
+  issues: write
+  pull-requests: write
+  actions: read
+
+jobs:
+  comment:
+    runs-on: ubuntu-latest
+    if: github.event.workflow_run.event == 'pull_request'
+    steps:
+      - name: Download scan artifact
+        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
+        with:
+          name: pr-risk-scan-results
+          run-id: ${{ github.event.workflow_run.id }}
+          github-token: ${{ github.token }}
+
+      - name: Upsert PR comment
+        uses: actions/github-script@f28e40c7f34bde8b3046d885e986cb6290c5673b # v7.1.0
+        with:
+          script: |
+            const fs = require('fs');
+            const marker = '<!-- pr-risk-scan-results -->';
+            const reportPath = 'report.md';
+            const prNumberPath = 'pr-number.txt';
+
+            if (!fs.existsSync(reportPath)) {
+              core.setFailed('Risk scan report.md artifact was not found.');
+              return;
+            }
+
+            const body = fs.readFileSync(reportPath, 'utf8');
+            let prNumber = null;
+
+            if (fs.existsSync(prNumberPath)) {
+              const parsed = parseInt(fs.readFileSync(prNumberPath, 'utf8').trim(), 10);
+              if (!Number.isNaN(parsed)) {
+                prNumber = parsed;
+              }
+            }
+
+            if (!prNumber) {
+              const fallback = context.payload.workflow_run.pull_requests?.[0]?.number;
+              if (fallback) {
+                prNumber = fallback;
+              }
+            }
+
+            if (!prNumber) {
+              core.setFailed('Could not determine PR number for comment upsert.');
+              return;
+            }
+
+            const { data: comments } = await github.rest.issues.listComments({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: prNumber,
+              per_page: 100,
+            });
+
+            const existing = comments.find((comment) => comment.body.includes(marker));
+
+            if (existing) {
+              await github.rest.issues.updateComment({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                comment_id: existing.id,
+                body,
+              });
+              console.log(`Updated existing risk scan comment ${existing.id}`);
+            } else {
+              await github.rest.issues.createComment({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                issue_number: prNumber,
+                body,
+              });
+              console.log('Created new risk scan comment');
+            }

.github/workflows/pr-risk-scan.yml

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+name: PR Risk Scan — Gate
+
+on:
+  pull_request:
+    branches: [staged]
+    types: [opened, synchronize, reopened]
+    paths:
+      - "skills/**"
+      - "agents/**"
+      - "workflows/**"
+      - "plugins/**"
+      - "hooks/**"
+      - "instructions/**"
+
+permissions:
+  contents: read
+  actions: read
+
+jobs:
+  scan:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+        with:
+          fetch-depth: 0
+
+      - name: Collect changed files
+        run: |
+          git diff --name-only --diff-filter=ACMR "origin/${{ github.base_ref }}...HEAD" > changed-files.txt
+          echo "Changed files:"
+          cat changed-files.txt || true
+
+      - name: Run PR risk scanner
+        run: |
+          mkdir -p pr-risk-results
+          node ./eng/pr-risk-scan.mjs \
+            --files changed-files.txt \
+            --output-json pr-risk-results/results.json \
+            --output-md pr-risk-results/report.md
+
+      - name: Save metadata
+        run: |
+          echo "${{ github.event.pull_request.number }}" > pr-risk-results/pr-number.txt
+
+      - name: Upload scan artifact
+        uses: actions/upload-artifact@bbbca2ddaa5d8feaa63e36b76fdaad77386f024f # v7
+        with:
+          name: pr-risk-scan-results
+          path: pr-risk-results/
+          retention-days: 1