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/e2e.yml

@@ -9,6 +9,9 @@ on:
     - cron: '0 6 * * *'  # Nightly at 6am UTC
   workflow_dispatch:
 
+permissions:
+  contents: read
+
 jobs:
   tier1:
     name: Tier 1 (Mechanical)
@@ -28,8 +31,8 @@ jobs:
           --health-timeout 5s
           --health-retries 5
     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
@@ -58,13 +61,13 @@ jobs:
           --health-timeout 5s
           --health-retries 5
     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
       - name: Install OpenClaw
-        run: npm install -g openclaw
+        run: npm install -g openclaw@2026.4.9
       - name: Configure OpenClaw MCP
         run: |
           mkdir -p ~/.openclaw