ci: harden GitHub Actions workflows (#185)

* Add GitHub Actions audit job (actionlint + zizmor) to CI

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

* Configure dependabot for GitHub Actions, bundler, and Docker

Batches all action updates into a single weekly PR. Adds cooldown
periods to all ecosystems.

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

* Add local GitHub Actions linting (actionlint + zizmor) to bin/setup and bin/ci

Install actionlint, shellcheck, and zizmor in bin/setup. Run both
linters as CI steps in config/ci.rb alongside existing style checks.

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

* Pin all GitHub Actions to SHA hashes

Run pinact to pin action versions to specific commit SHAs,
preventing supply chain attacks from tag mutation.

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

* Fix high severity zizmor findings

- Suppress unpinned-images for redis service containers (digest
  pinning is nontrivial for service containers)
- Move workflow-level permissions to job-level in publish-image.yml
  (build gets full set, manifest gets only what it needs)

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

* Fix medium severity zizmor findings

- Add persist-credentials: false to all checkout steps
- Add permissions: {} at workflow level in ci.yml
- Add job-level permissions (contents: read) to all CI jobs

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

* Fix informational template-injection findings in publish-image.yml

Move steps.meta.outputs.tags from inline ${{ }} expressions to env
vars in both the manifest creation and cosign signing steps.

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

* Update brakeman to 8.0.4

bin/brakeman uses --ensure-latest which fails if not on the newest version.

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

---------

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

bin/setup

@@ -100,6 +100,20 @@ if ! redis_running; then
   fi
 fi
 
+# Install GitHub Actions linting tools
+for tool in actionlint shellcheck zizmor; do
+  if ! command -v "$tool" &> /dev/null; then
+    if command -v brew &> /dev/null; then
+      step "Installing $tool" brew install "$tool"
+    elif command -v pacman &> /dev/null; then
+      step "Installing $tool" sudo pacman -S --noconfirm "$tool"
+    else
+      echo "Error: install $tool manually" >&2
+      exit 1
+    fi
+  fi
+done
+
 step "Cleaning up logs and tempfiles" rails log:clear tmp:clear
 
 step "Restarting services" rails restart