chore(ci): improve repository maintenance configuration

- Enable weekly Dependabot updates for GitHub Actions dependencies
- Consolidate duplicate Trivy scans into a single SARIF scan with exit-code gate
- Ignore macOS metadata and locally downloaded drone-ssh binaries
- Add feature request issue template and pull request checklist template

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,6 @@
+# macOS
+.DS_Store
+
+# drone-ssh binaries downloaded by entrypoint.sh during local testing
+drone-ssh-*
+checksums.txt