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>

internal/build/manifest.go

@@ -121,6 +121,11 @@ func ValidatePluginID(id string) error {
 	return nil
 }
 
+// FindAppDir walks up from start looking for summer.yaml.
+func FindAppDir(start string) (string, error) {
+	return findAppDir(start)
+}
+
 func findAppDir(start string) (string, error) {
 	if start == "" {
 		return "", fmt.Errorf("build: app directory is empty")