When a host transitions from unreachable → online via heartbeat, trigger
ReconcileHost in a background goroutine so "missing" sandboxes are
resolved instantly instead of waiting up to 60s for the next monitor tick.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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.
