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.

internal/api/handlers_audit.go

@@ -8,9 +8,9 @@ import (
 
 	"github.com/jackc/pgx/v5/pgtype"
 
-	"git.omukk.dev/wrenn/wrenn/internal/auth"
-	"git.omukk.dev/wrenn/wrenn/internal/id"
-	"git.omukk.dev/wrenn/wrenn/internal/service"
+	"git.omukk.dev/wrenn/wrenn/pkg/auth"
+	"git.omukk.dev/wrenn/wrenn/pkg/id"
+	"git.omukk.dev/wrenn/wrenn/pkg/service"
 )
 
 type auditHandler struct {