fix: expanduser in project_root, remove redundant cwd, add tests

- get_myclaude_project_root() now expands ~ in MYCLAUDE_PROJECT_ROOT
  (was resolving to ./~/path instead of $HOME/path)
- Extract hardcoded 16/8 depth limits as _MAX_WALK_DEPTH / _MAX_SOURCE_DEPTH
- Remove redundant cwd= from _run_pip_editable (already specified via -e)
- Add test_cwd_invalid_directory and test_env_var_expands_tilde
- Remove incorrect test_extra_args_forwarded (click.Group does not
  support forwarding arbitrary args; ctx.args is always empty)

tests/test_cli.py

@@ -99,3 +99,12 @@ class TestCliDefaultBehavior:
             assert result.exit_code == 0
             call_args = mock_run.call_args[0][0]
             assert "--dangerously-skip-permissions" in call_args
+
+    def test_cwd_invalid_directory(self, tmp_path: Path) -> None:
+        """--cwd pointing to non-existent directory should error."""
+        runner = CliRunner()
+        bad_dir = tmp_path / "does_not_exist"
+
+        result = runner.invoke(cli, ["--cwd", str(bad_dir)])
+        assert result.exit_code == 1
+        assert "not a directory" in result.output.lower()