feat: Add supply chain security improvements

- Pin all GitHub Actions to specific commit SHAs for immutability
- Add cargo-audit job to CI pipeline for vulnerability scanning
- Create SECURITY.md with vulnerability reporting guidelines
- Add Dependabot configuration for automated dependency updates
  - Weekly updates for both Cargo and GitHub Actions
  - Grouped patch and minor updates for Cargo dependencies
- Update job dependencies to run security checks before integration tests

This improves the project's security posture by:
1. Preventing action hijacking through SHA pinning
2. Catching known vulnerabilities early with cargo-audit
3. Providing clear security reporting channels
4. Automating dependency updates to reduce security debt

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

.github/dependabot.yml

@@ -0,0 +1,46 @@
+version: 2
+updates:
+  # Enable version updates for Rust dependencies
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+      time: "08:00"
+    open-pull-requests-limit: 10
+    reviewers:
+      - "imjasonh"
+    labels:
+      - "dependencies"
+      - "rust"
+    commit-message:
+      prefix: "chore"
+      include: "scope"
+    groups:
+      patch-updates:
+        patterns:
+          - "*"
+        update-types:
+          - "patch"
+      minor-updates:
+        patterns:
+          - "*"
+        update-types:
+          - "minor"
+
+  # Enable version updates for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+      time: "08:00"
+    open-pull-requests-limit: 5
+    reviewers:
+      - "imjasonh"
+    labels:
+      - "dependencies"
+      - "github-actions"
+    commit-message:
+      prefix: "chore"
+      include: "scope"