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/templates.sql

@@ -1,6 +1,6 @@
 -- name: InsertTemplate :one
-INSERT INTO templates (id, name, type, vcpus, memory_mb, size_bytes, team_id, default_user, default_env)
-VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9)
+INSERT INTO templates (id, name, type, vcpus, memory_mb, size_bytes, team_id, default_user, default_env, metadata)
+VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10)
 RETURNING *;
 
 -- name: GetTemplate :one