feat(03-01): add shared postgres pool and plugin migrations

Open one pgx stdlib *sql.DB, hand it to GORM, and run per-plugin
gormigrate sets with isolated history tables after an ICU pl-PL check.

Co-authored-by: Cursor <cursoragent@cursor.com>

pact/capabilities.go

@@ -4,6 +4,7 @@ import (
 	"io/fs"
 
 	"git.golem15.com/golem15/summercms/bonfire"
+	"github.com/go-gormigrate/gormigrate/v2"
 )
 
 // HasCommands is implemented by plugins that register console commands.
@@ -18,19 +19,24 @@ type HasConfig interface {
 	ConfigFS() fs.FS
 }
 
+// HasMigrations is implemented by plugins that ship an ordered gormigrate
+// set. The kernel runs each set in party.Activate order with a separate
+// history table per plugin ID.
+type HasMigrations interface {
+	Migrations() []*gormigrate.Migration
+}
+
 // OptionalMessage is a service an optional plugin may publish so other
 // plugins can integrate without importing that plugin's package.
 type OptionalMessage interface {
 	Message() string
 }
 
-// Future KERN-03 capability families are type-asserted when their first
-// consumer packages exist. Method payloads are not declared here so pact
-// does not import surf, lagoon, festival, or other packages that Phase 1
-// does not yet ship:
+// Future capability families are type-asserted when their first consumer
+// packages exist. Method payloads for HTTP/model surfaces are declared in
+// the phase that ships surf:
 //
 //	HasModels
-//	HasMigrations
 //	HasRoutes
 //	HasMiddleware
 //	HasJobs
@@ -42,5 +48,5 @@ type OptionalMessage interface {
 //	HasMailTemplates
 //	HasLang
 //
-// Phase 1 type-asserts only HasConfig (party, before Register) and
-// HasCommands (generated app main, after Boot).
+// The kernel type-asserts HasConfig (party, before Register), HasCommands
+// (generated app main, after Boot), and HasMigrations (lagoon migrate).