refactor: use project-named state file and store prompt/params directly

- State filename now derives from config: cards.bulkgen.yaml produces
  .cards.bulkgen-state.yaml instead of .bulkgen.state.yaml
- Store resolved prompt text and extra params directly in state file
  instead of hashing them, making state files human-readable
- Only file input contents remain hashed (SHA-256)
- Thread project_name through builder and CLI
- Remove hash_string() and _extra_hash() helpers
- Update .gitignore pattern to .*.bulkgen-state.yaml

tests/test_cli.py

@@ -132,7 +132,8 @@ class TestBuildCommand:
 
         assert result.exit_code == 0
         call_args = mock_run.call_args
-        assert call_args[0][2] == "output.txt"
+        # positional args: (config, project_dir, project_name, target)
+        assert call_args[0][3] == "output.txt"
 
 
 class TestCleanCommand:
@@ -141,7 +142,8 @@ class TestCleanCommand:
     def test_clean_removes_targets(self, cli_project: Path) -> None:
         _ = (cli_project / "output.txt").write_text("generated")
         _ = (cli_project / "image.png").write_bytes(b"\x89PNG")
-        _ = (cli_project / ".bulkgen.state.yaml").write_text("targets: {}")
+        state_file = ".project.bulkgen-state.yaml"
+        _ = (cli_project / state_file).write_text("targets: {}")
 
         with patch("bulkgen.cli.Path") as mock_path_cls:
             mock_path_cls.cwd.return_value = cli_project
@@ -150,7 +152,7 @@ class TestCleanCommand:
         assert result.exit_code == 0
         assert not (cli_project / "output.txt").exists()
         assert not (cli_project / "image.png").exists()
-        assert not (cli_project / ".bulkgen.state.yaml").exists()
+        assert not (cli_project / state_file).exists()
         assert "Cleaned 2 artifact(s)" in result.output
 
     def test_clean_no_artifacts(self, cli_project: Path) -> None: