ci: add Trivy vulnerability scanning

.github/workflows/trivy.yml

@@ -0,0 +1,95 @@
+name: Trivy Security Scan
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+  schedule:
+    # Weekly full scan, Mondays 06:00 UTC.
+    - cron: "0 6 * * 1"
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+# Cancel superseded PR runs; non-PR runs get a unique group so they are never
+# interrupted (mirrors ci.yml).
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  # Blocking gate on PRs and pushes to main: fail only on fixable CRITICAL/HIGH
+  # vulnerabilities in Go dependencies, so a red check always has an actionable
+  # fix (bump the dependency). Vulns with no upstream fix don't wedge every PR.
+  scan-pr:
+    name: Dependency Scan
+    if: github.event_name == 'pull_request' || github.event_name == 'push'
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    permissions:
+      contents: read
+      security-events: write # upload SARIF to code scanning
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v7
+
+      # SARIF report for the Security tab covers all severities and never fails
+      # the job (the gate below owns pass/fail). trivy-action ignores severity
+      # filters when format is sarif, so this cannot double as the gate.
+      - name: Run Trivy (SARIF report)
+        uses: aquasecurity/trivy-action@v0.36.0
+        with:
+          scan-type: fs
+          scanners: vuln
+          format: sarif
+          output: trivy-results.sarif
+
+      # Fork-PR tokens have no security-events:write, so skip the upload there
+      # (the scan + gate still run).
+      - name: Upload SARIF to code scanning
+        if: always() && github.event.pull_request.head.repo.fork != true
+        uses: github/codeql-action/upload-sarif@v3
+        with:
+          sarif_file: trivy-results.sarif
+
+      # The blocking gate: fail only on fixable CRITICAL/HIGH vulnerabilities.
+      - name: Run Trivy (fixable CRITICAL/HIGH gate)
+        uses: aquasecurity/trivy-action@v0.36.0
+        with:
+          scan-type: fs
+          scanners: vuln
+          severity: CRITICAL,HIGH
+          ignore-unfixed: true
+          exit-code: "1"
+          format: table
+
+  # Weekly full scan: every severity, including vulns with no fix yet. New CVEs
+  # land against unchanged code, so this catches what the PR gate never sees.
+  # Results go to the Security tab (code scanning) — watchers subscribed to
+  # security alerts get notified there, no Slack/webhook plumbing.
+  scan-weekly:
+    name: Full Scan
+    if: github.event_name == 'schedule' || github.event_name == 'workflow_dispatch'
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    permissions:
+      contents: read
+      security-events: write # upload SARIF to code scanning
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v7
+
+      - name: Run Trivy (all severities)
+        uses: aquasecurity/trivy-action@v0.36.0
+        with:
+          scan-type: fs
+          scanners: vuln
+          format: sarif
+          output: trivy-results.sarif
+
+      - name: Upload SARIF to code scanning
+        uses: github/codeql-action/upload-sarif@v3
+        with:
+          sarif_file: trivy-results.sarif