fix(sqlite): emit inline foreign keys, bare-name default schema, direct exec

SQLite can't ALTER TABLE ADD CONSTRAINT, so foreign keys are now written
as inline FOREIGN KEY clauses in CREATE TABLE instead of commented-out
ALTER statements. The default schema (public/main) now produces bare
table names instead of a "public_" prefix; other schemas are still
prefixed to avoid collisions. Also adds direct-to-file execution: the
sqlite writer can now apply generated DDL straight to a .db file via
Metadata["connection_string"], wired into `relspec merge --output-conn`.

pkg/writers/sqlite/writer_test.go

@@ -85,8 +85,11 @@ func TestWriteDatabase(t *testing.T) {
 		t.Error("Expected CREATE TABLE statement")
 	}
 
-	if !strings.Contains(output, "\"public_users\"") {
-		t.Error("Expected flattened table name public_users")
+	if !strings.Contains(output, "\"users\"") {
+		t.Error("Expected bare table name users (default schema should not be prefixed)")
+	}
+	if strings.Contains(output, "\"public_users\"") {
+		t.Error("Did not expect flattened table name public_users for the default public schema")
 	}
 
 	if !strings.Contains(output, "INTEGER PRIMARY KEY AUTOINCREMENT") {
@@ -322,13 +325,15 @@ func TestWriteSchema_MultiSchema(t *testing.T) {
 
 	output := buf.String()
 
-	// Check for flattened table names from both schemas
+	// Non-default schemas are still prefixed to avoid name collisions...
 	if !strings.Contains(output, "\"auth_sessions\"") {
 		t.Error("Expected flattened table name auth_sessions")
 	}
 
-	if !strings.Contains(output, "\"public_posts\"") {
-		t.Error("Expected flattened table name public_posts")
+	// ...but the default "public" schema is not, since it's typically the
+	// only schema and bare names read better (and match e.g. DBML output).
+	if !strings.Contains(output, "\"posts\"") {
+		t.Error("Expected bare table name posts")
 	}
 }