feat(devenv): add containerized development environment (#116)

* feat(devenv): add containerized development environment

Replace local-process dev workflow with a fully containerized stack
using docker compose. This enables consistent development across
machines without requiring local Go/Node toolchains or containerd.

- Add Dockerfile.server.dev with containerd + CNI networking support
- Add Dockerfile.web.dev for frontend dev server
- Add server-dev-entrypoint.sh for containerd lifecycle management
- Expand devenv/docker-compose.yml with server, agent, web, migrate
  and deps services with proper health checks and dependency ordering
- Update app.dev.toml to use container service names instead of localhost
- Refactor mise.toml dev tasks to drive docker compose workflow
- Support agent_gateway.server_addr in config package for inter-container
  communication

* feat(devenv): add hot-reload and registry mirror support

- Add air for Go server hot-reload in dev containers
- Fix agent_gateway host in dev config (0.0.0.0 -> agent)
- Add configurable registry mirror for China mainland users
- Unify MCP image refs via MCPConfig.ImageRef()

* feat(scripts): add China mainland mirror option to install script

Prompt users to opt-in to memoh.cn mirror during installation,
which applies docker-compose.cn.yml overlay and sets registry
in config.toml for MCP image pulls.

internal/mcp/manager.go

@@ -338,10 +338,7 @@ func (m *Manager) dataMount() string {
 }
 
 func (m *Manager) imageRef() string {
-	if m.cfg.Image != "" {
-		return m.cfg.Image
-	}
-	return config.DefaultMCPImage
+	return m.cfg.ImageRef()
 }
 
 func validateBotID(botID string) error {