docs: rewrite README with comprehensive toolchain usage guide

.gitignore

@@ -0,0 +1,32 @@
+# --- Secrets / signing (must never be committed) ---
+*.keystore
+*.jks
+*.p12
+*.pem
+*.key
+keystore*
+*.base64
+key.properties
+local.properties
+
+# --- Build / scratch ---
+*.zip
+*.tmp
+tmp/
+out/
+dist/
+
+# --- OS cruft ---
+.DS_Store
+Thumbs.db
+desktop.ini
+
+# --- Editors / IDEs ---
+.idea/
+.vscode/
+*.swp
+*.swo
+*~
+
+# --- Logs ---
+*.log