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/migrations/20260415134310_add_metadata.sql

@@ -0,0 +1,9 @@
+-- +goose Up
+ALTER TABLE sandboxes ADD COLUMN metadata JSONB NOT NULL DEFAULT '{}';
+ALTER TABLE templates ADD COLUMN metadata JSONB NOT NULL DEFAULT '{}';
+ALTER TABLE template_builds ADD COLUMN metadata JSONB NOT NULL DEFAULT '{}';
+
+-- +goose Down
+ALTER TABLE sandboxes DROP COLUMN metadata;
+ALTER TABLE templates DROP COLUMN metadata;
+ALTER TABLE template_builds DROP COLUMN metadata;