feat: [SEC-7263] Add dependency-scan GitHub Actions workflow (#10)

# feat: [SEC-7263] Add dependency-scan GitHub Actions workflow

## Summary
Adds automated dependency scanning and license policy evaluation as part
of LaunchDarkly's security initiative SEC-7263. This workflow generates
Software Bill of Materials (SBOM) for Node.js dependencies and evaluates
them against security policies.

**Key changes:**
- New `.github/workflows/dependency-scan.yml` with two jobs: SBOM
generation and policy evaluation
- Configured for Node.js dependency scanning only
- Runs on all pull requests and pushes to main branch
- Uses private repository workflow pattern with
`launchdarkly/common-actions`

## Review & Testing Checklist for Human
- [ ] **Verify workflow permissions**: Test that the workflow runs
without "Unable to resolve action" or 403 permission errors when
accessing `launchdarkly/common-actions`
- [ ] **Validate SBOM generation**: Check that generated SBOM artifacts
contain expected Node.js dependencies from
package.json/package-lock.json
- [ ] **Test policy evaluation**: Confirm the evaluate-policy job
correctly processes SBOM files and reports any license violations or
security issues
- [ ] **Confirm repository type**: Verify that Node.js-only scanning
(`types: 'nodejs'`) is appropriate - check if this repo contains Go,
Python, or other language dependencies that should also be scanned

**Recommended test plan**: Create a test PR to trigger the workflow and
verify both jobs complete successfully with proper SBOM artifacts
generated.

### Notes
- This is part of the broader SEC-7263 security initiative to implement
dependency scanning across all LaunchDarkly npm ecosystem repositories
- Workflow uses pinned SHA for security:
`runs-on/action@cd2b598b0515d39d78c38a02d529db87d2196d1e`
- Requested by Patrick Kaeding (@pkaeding) 
- Link to Devin run:
https://app.devin.ai/sessions/434bb14b7bac4d81b9979b88965be92b

[SEC-7263]:
https://launchdarkly.atlassian.net/browse/SEC-7263?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: kparkinson-ld

.github/workflows/dependency-scan.yml

@@ -0,0 +1,15 @@
+name: Dependency Scan
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  dependency-scan:
+    uses: launchdarkly/common-workflows/.github/workflows/dependency-scan.yml@main
+    with:
+      types: 'nodejs'
+      runs-on: 'ubuntu-latest'
+    secrets: inherit