Add development setup guide + shared VS Code config

New docs/DEVELOPMENT-SETUP.md: clone-to-first-change onboarding
(Docker fast path, manual venv+Node daily driver, VS Code, the dev
loop, first-change pointer at the plugin lab, troubleshooting). Ship
.vscode/ launch/tasks/extensions so F5 debugs the backend on 5001 and
a task runs both servers; personal settings.json stays ignored. Fix
the README manual path - it ran the backend on the default 5000, but
the frontend dev server proxies to 5001, so nothing loaded; also add
the plugin upgrade-all step and a VS Code pointer.

.gitignore

@@ -28,7 +28,11 @@ env/
 
 # IDE
 .idea/
-.vscode/
+.vscode/*
+# Share the team's launch/tasks/extensions; keep personal settings out.
+!.vscode/launch.json
+!.vscode/tasks.json
+!.vscode/extensions.json
 *.swp
 *.swo
 *~