feat: auto-register extension dev servers with host

[dschmidt]

Summary

Remote extension dev servers automatically register themselves with the host dev server on startup — no more manual config.json editing for dev.

• Host (vite.config.ts): REST API at /_dev/apps (POST/DELETE/GET), health check every 10s, merges registered remotes into /config.json proxy response, triggers browser reload via HMR
• Remote (extension-sdk): Registration plugin posts to host on httpServer.listening, watches src/manifest.json and src/config.json for changes and re-registers
• Config: Reads manifest.json defaults + config.json overrides with deep merge (matches Go-side mergo behavior)
• Cleanup: Host health check removes unreachable remotes within 10s

How it works

1. Start host: pnpm vite (port 9201)
2. Start any extension: pnpm vite (any port)
3. Extension auto-registers → browser reloads → app appears
4. Stop extension → health check cleans up → browser reloads → app gone
5. Edit src/manifest.json or src/config.json → re-registers → browser reloads with new config

Configuration

Option	Default	Description
overrides.hostUrl	https://host.docker.internal:9201	Host dev server URL
OPENCLOUD_WEB_HOST_URL env	(same)	Alternative to override in code

Debug

curl -sk https://host.docker.internal:9201/_dev/apps

Test plan

• Start host, then start extension → app auto-appears
• Edit manifest.json config → browser reloads with updated config
• Stop extension (Ctrl+C) → health check removes it within 10s
• Start extension before host → graceful failure, works after host starts + reload
• Multiple extensions on different ports simultaneously

[dschmidt]

A few minor things to improve:

 const hostUrl =
      overrides?.hostUrl ||
      process.env.OPENCLOUD_WEB_HOST_URL ||
      'https://host.docker.internal:9201'

These names aren't great.

I noticed that we detect when remotes/apps become stale, but if the Web Vite instance is restarted - apps never reregister..