security: pin GitHub Actions, add gitleaks CI, harden permissions (v0.4.2) (#23)

* security: pin GitHub Actions to commit SHAs, add gitleaks CI

- Pin all 5 actions (checkout, setup-bun, upload-artifact, download-artifact,
  action-gh-release) to commit SHAs across 3 workflow files
- Add permissions: contents: read to test.yml and e2e.yml
- Add gitleaks secret scanning job to test.yml
- Pin openclaw install to v2026.4.9 in e2e.yml

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* security: add .gitleaks.toml config

Allowlists test fixtures, example env files, and skill documentation
to prevent false positives from the gitleaks CI step.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: add GitHub Actions SHA maintenance rule to CLAUDE.md

Instructs /ship and /review to check for stale SHA pins and update
them, keeping action versions fresh without manual effort.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: add S3 Sig V4 TODO from CSO audit

Deferred from security audit. S3 storage backend accepts credentials
but sends unsigned requests. Implement when S3 becomes a real
deployment path.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* chore: bump version and changelog (v0.4.2)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/release.yml

@@ -20,14 +20,14 @@ jobs:
             artifact: gbrain-linux-x64
     runs-on: ${{ matrix.os }}
     steps:
-      - uses: actions/checkout@v4
-      - uses: oven-sh/setup-bun@v2
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5  # v4
+      - uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6  # v2
         with:
           bun-version: latest
       - run: bun install
       - run: bun test
       - run: bun build --compile --target=${{ matrix.target }} --outfile bin/${{ matrix.artifact }} src/cli.ts
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02  # v4
         with:
           name: ${{ matrix.artifact }}
           path: bin/${{ matrix.artifact }}
@@ -36,11 +36,11 @@ jobs:
     needs: build
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/download-artifact@v4
+      - uses: actions/download-artifact@d3f86a106a0bac45b974a628896c90dbdf5c8093  # v4
         with:
           path: artifacts
       - name: Create release
-        uses: softprops/action-gh-release@v2
+        uses: softprops/action-gh-release@153bb8e04406b158c6c84fc1615b65b24149a1fe  # v2
         with:
           files: |
             artifacts/gbrain-darwin-arm64/gbrain-darwin-arm64