fix: OS release prep - bootstrap script, deps, docs

- Fix bootstrap script (.env.example.local → .env.example)
- Fix @langchain/core version conflict (^1.0.1 → ^0.3.0)
- Add settings:open event listener for LocalKeyGate button
- Pin Next.js to 15.1.3 (was "latest")
- Default runtime to 'local' mode
- Remove desktop app references from UI text
- Rewrite MCP docs for web-only context
- Add SECURITY.md, CODE_OF_CONDUCT.md, CHANGELOG.md
- Add docs/README.md, docs/TROUBLESHOOTING.md
- Update README with platform support table

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/config/runtime.ts

@@ -1,4 +1,4 @@
-const rawDeploymentMode = (process.env.NEXT_PUBLIC_DEPLOYMENT_MODE || 'cloud').toLowerCase();
+const rawDeploymentMode = (process.env.NEXT_PUBLIC_DEPLOYMENT_MODE || 'local').toLowerCase();
 const backendFlagEnabled = process.env.NEXT_PUBLIC_ENABLE_SUBSCRIPTION_BACKEND === 'true';
 
 export type DeploymentMode = 'local' | 'cloud';