APPSEC-553: Add README, Semgrep OSS, and scan for secrets workflows

.github/workflows/scan-for-secrets.yml

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+name: Scan for Secrets
+
+on:
+  pull_request: {}
+  merge_group:
+    types: [checks_requested]
+  workflow_dispatch: {}
+
+jobs:
+  scan_for_secrets:
+    name: public
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    # Skip Dependabot PRs and auto-pass in merge queue (base branch already scanned)
+    if: |
+      github.actor != 'dependabot[bot]' &&
+      github.actor != 'github-actions[bot]'
+    steps:
+      - name: Establish event metadata
+        id: establish_metadata
+        shell: bash
+        env:
+          EVENT_NAME: ${{ github.event_name }}
+          PR_COMMITS: ${{ github.event.pull_request.commits }}
+          PR_HEAD_REF: ${{ github.event.pull_request.head.ref }}
+        run: |
+          if [[ "$EVENT_NAME" == "pull_request" ]]; then
+            echo "fetch_depth=$(($PR_COMMITS + 2))" >> $GITHUB_OUTPUT
+            echo "branch=$PR_HEAD_REF" >> $GITHUB_OUTPUT
+          fi
+
+      - name: Auto-pass in merge queue
+        if: github.event_name == 'merge_group'
+        run: echo 'Auto-passing in merge queue'
+
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        if: github.event_name == 'pull_request'
+        with:
+          ref: ${{ steps.establish_metadata.outputs.branch }}
+          fetch-depth: ${{ steps.establish_metadata.outputs.fetch_depth }}
+
+      - name: Scan for secrets
+        if: github.event_name == 'pull_request'
+        uses: trufflesecurity/trufflehog@939f053fc5cc13136efeb9e4d505051455d135dd
+        with:
+          extra_args: --only-verified

.github/workflows/semgrep.yml

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+name: Semgrep OSS
+
+on:
+  pull_request: {}
+  workflow_dispatch: {}
+  merge_group:
+    types: [checks_requested]
+  schedule:
+    # Full scan of main every Monday at 06:17 UTC.
+    # Randomized time to avoid GHA load spikes.
+    - cron: '17 6 * * 1'
+
+jobs:
+  semgrep:
+    name: public
+    runs-on: ubuntu-latest
+    container:
+      image: semgrep/semgrep:1.161.0@sha256:326e5f41cc972bb423b764a14febbb62bbad29ee1c01820805d077dd868fea48
+    permissions:
+      contents: read
+      security-events: write  # Required for SARIF upload to GitHub Code Scanning
+      actions: read
+    # Skip Dependabot PRs and merge group events (diff scan not useful pre-merge)
+    if: |
+      github.actor != 'dependabot[bot]' &&
+      github.actor != 'github-actions[bot]' &&
+      github.event_name != 'merge_group'
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+
+      - name: Run Semgrep OSS (diff-aware)
+        # PR scan: diff-aware via semgrep ci, only surfaces new findings against the merge base.
+        # No SEMGREP_APP_TOKEN = fully local mode; no data leaves the runner.
+        if: github.event_name == 'pull_request'
+        run: semgrep ci --sarif --output=semgrep.sarif --config=auto
+        continue-on-error: true
+
+      - name: Run Semgrep OSS (full scan)
+        # Scheduled/manual scan: full repo scan via semgrep scan.
+        if: github.event_name != 'pull_request'
+        run: semgrep scan --sarif --output=semgrep.sarif --config=auto
+        continue-on-error: true
+
+      - name: Upload findings to GitHub Code Scanning
+        uses: github/codeql-action/upload-sarif@ff0a06e83cb2de871e5a09832bc6a81e7276941f # v3.28.18
+        with:
+          sarif_file: semgrep.sarif
+          category: semgrep-oss

README.md

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+# security-workflows-public
+
+Public AppSec tooling from Gemini's security engineering team, adapted for use in open source projects.
+
+## What's Here
+
+This repository contains security workflows, scanners, and automation tools for securing Gemini's open source projects.
+
+## Workflows
+
+### Semgrep OSS
+[`.github/workflows/semgrep.yml`](.github/workflows/semgrep.yml)
+
+Runs [Semgrep](https://semgrep.dev) static analysis on pull requests and on a weekly schedule. Uses community rulesets (`p/default`, `p/security-audit`, `p/owasp-top-ten`) with no Semgrep account or token required. Findings are uploaded to GitHub Code Scanning.
+
+### Scan for Secrets
+[`.github/workflows/scan-for-secrets.yml`](.github/workflows/scan-for-secrets.yml)
+
+Scans pull requests for verified secrets using [TruffleHog](https://github.com/trufflesecurity/trufflehog). Only reports verified findings to minimize noise. No token or license required.
+
+## Contributing
+
+This repository is maintained by Gemini's AppSec team. External contributions and feedback are welcome via GitHub Issues.
+
+## License
+
+See [LICENSE](LICENSE) for details.