feat: dev env bootstrap, deferred sessions, and mock agent fallback

- Add dotenv loader for NODE_ENV=development with .env.development.example
- Add factory self-hosting deployment docs
- Defer agent session creation in handoff init (create on first prompt)
- Fall back to mock agent when Claude/Codex credentials are missing
- Replace stub mock agent launcher with real ACP JSON-RPC Node.js script
- Fix React useEffect dependency array in org settings
- Clean up tracked .context cache/temp files

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.development.example

@@ -0,0 +1,23 @@
+# Load this file only when NODE_ENV=development.
+# The backend does not load dotenv files in production.
+
+APP_URL=http://localhost:4173
+BETTER_AUTH_URL=http://localhost:4173
+BETTER_AUTH_SECRET=sandbox-agent-factory-development-only-change-me
+GITHUB_REDIRECT_URI=http://localhost:4173/api/rivet/app/auth/github/callback
+
+# Fill these in when enabling live GitHub OAuth.
+GITHUB_CLIENT_ID=
+GITHUB_CLIENT_SECRET=
+
+# Fill these in when enabling GitHub App-backed org installation and repo import.
+GITHUB_APP_ID=
+GITHUB_APP_CLIENT_ID=
+GITHUB_APP_CLIENT_SECRET=
+GITHUB_APP_PRIVATE_KEY=
+
+# Fill these in when enabling live Stripe billing.
+STRIPE_SECRET_KEY=
+STRIPE_PUBLISHABLE_KEY=
+STRIPE_WEBHOOK_SECRET=
+