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>

examples/hello/hello_test.go

@@ -104,6 +104,14 @@ func TestBuiltBinaryGreeterHello(t *testing.T) {
 	assertGreeting(t, got, want)
 }
 
+func TestMigrateWithoutDSNFailsLoudly(t *testing.T) {
+	var buf bytes.Buffer
+	err := run([]string{"migrate"}, &buf)
+	if err == nil || !strings.Contains(err.Error(), "database.dsn") {
+		t.Fatalf("want empty DSN error, got %v\n%s", err, buf.String())
+	}
+}
+
 func TestBuiltBinaryUnknownCommandExitsNonZero(t *testing.T) {
 	runSummerBuild(t)
 	cmd := exec.Command("./bin/hello", "does:not-exist")