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.

pkg/auth/oauth/provider.go

@@ -0,0 +1,41 @@
+package oauth
+
+import "context"
+
+// UserProfile is the normalized user info returned by an OAuth provider.
+type UserProfile struct {
+	ProviderID string
+	Email      string
+	Name       string
+}
+
+// Provider abstracts an OAuth 2.0 identity provider.
+type Provider interface {
+	// Name returns the provider identifier (e.g. "github", "google").
+	Name() string
+	// AuthCodeURL returns the URL to redirect the user to for authorization.
+	AuthCodeURL(state string) string
+	// Exchange trades an authorization code for a user profile.
+	Exchange(ctx context.Context, code string) (UserProfile, error)
+}
+
+// Registry maps provider names to Provider implementations.
+type Registry struct {
+	providers map[string]Provider
+}
+
+// NewRegistry creates an empty provider registry.
+func NewRegistry() *Registry {
+	return &Registry{providers: make(map[string]Provider)}
+}
+
+// Register adds a provider to the registry.
+func (r *Registry) Register(p Provider) {
+	r.providers[p.Name()] = p
+}
+
+// Get looks up a provider by name.
+func (r *Registry) Get(name string) (Provider, bool) {
+	p, ok := r.providers[name]
+	return p, ok
+}