Refactored to maintain a separate cloud version

Moves 12 packages from internal/ to pkg/ (config, id, validate, events, db,
auth, lifecycle, scheduler, channels, audit, service) so they can be imported
by the enterprise repo as a Go module dependency.

Introduces pkg/cpextension (shared Extension interface + ServerContext) and
pkg/cpserver (Run() entrypoint with functional options) so the enterprise
main.go can call cpserver.Run(cpserver.WithExtensions(...)) without duplicating
the 20-step server bootstrap. Adds db/migrations/embed.go for go:embed access
to OSS SQL migrations from the enterprise module.

cmd/control-plane/main.go is reduced to a 10-line wrapper around cpserver.Run.

envd/internal/api/init_test.go

@@ -79,7 +79,7 @@ func newTestAPI(accessToken *SecureToken, mmdsClient MMDSClient) *API {
 	defaults := &execcontext.Defaults{
 		EnvVars: utils.NewMap[string, string](),
 	}
-	api := New(&logger, defaults, nil, false, context.Background(), nil)
+	api := New(&logger, defaults, nil, false, context.Background(), nil, "test")
 	if accessToken != nil {
 		api.accessToken.TakeFrom(accessToken)
 	}