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.

db/queries/template_builds.sql

@@ -34,5 +34,5 @@ WHERE id = $1;
 
 -- name: UpdateBuildDefaults :exec
 UPDATE template_builds
-SET default_user = $2, default_env = $3
+SET default_user = $2, default_env = $3, metadata = $4
 WHERE id = $1;