docs(ai): improve onboarding steps and add devcontainer

- Update README.md to recommend `npm ci` and include test steps.
- Update CONTRIBUTING.md with explicit development setup instructions.
- Add `.devcontainer/devcontainer.json` for reproducible Node 20 environment.
- Add audit report in `ai/reports/onboarding-20260118.md`.

.devcontainer/devcontainer.json

@@ -0,0 +1,13 @@
+{
+	"name": "bitvid",
+	"image": "mcr.microsoft.com/devcontainers/javascript-node:20",
+	"postCreateCommand": "npm ci",
+	"customizations": {
+		"vscode": {
+			"extensions": [
+				"esbenp.prettier-vscode",
+				"bradlc.vscode-tailwindcss"
+			]
+		}
+	}
+}