chore: add GitHub automation and fix rand vulnerability

GitHub Actions:
- Add Dependabot for weekly dependency updates (cargo, github-actions)
- Add PR labeler workflow with file-based auto-labeling
- Add issue/PR templates (bug report, feature request, PR template)
- Fix release.yml artifact paths and labeler.yml config syntax
Security:
- Upgrade rand 0.8 → 0.10 to fix RUSTSEC-2026-0097 unsoundness
- Update screenshot.rs for rand 0.10 API (thread_rng → rng, gen_range → random_range)

.github/labeler.yml

@@ -0,0 +1,23 @@
+documentation:
+  - changed-files:
+      - any-glob-to-any-file: ['README.md', '*.md', 'docs/**']
+
+features:
+  - changed-files:
+      - any-glob-to-any-file: ['src/input.rs', 'src/lock.rs', 'src/config.rs']
+
+rendering:
+  - changed-files:
+      - any-glob-to-any-file: ['src/render/**', 'src/render.rs']
+
+ci:
+  - changed-files:
+      - any-glob-to-any-file: ['.github/**', '.github/workflows/**']
+
+dependencies:
+  - changed-files:
+      - any-glob-to-any-file: ['Cargo.toml', 'Cargo.lock']
+
+refactoring:
+  - changed-files:
+      - any-glob-to-any-file: ['src/util.rs', 'src/system.rs']