docs: initial design documents

- 9 pages: index, architecture, capabilities, roadmap, security,
  maintenance, pm-review, backlog, flow
- shared style.css (dark theme)
- .gitignore for OS / editor / build artifacts

.gitignore

@@ -0,0 +1,38 @@
+# OS
+.DS_Store
+Thumbs.db
+desktop.ini
+
+# Editor
+.vscode/
+.idea/
+*.swp
+*.swo
+*~
+
+# Logs
+*.log
+npm-debug.log*
+
+# Env
+.env
+.env.local
+.env.*.local
+
+# Build artifacts
+node_modules/
+dist/
+build/
+target/
+*.exe
+*.deb
+*.apk
+*.dmg
+
+# Tauri
+src-tauri/target/
+
+# Expo / React Native
+.expo/
+.expo-shared/
+*.tsbuildinfo