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)

bin/commands/update.py

@@ -23,7 +23,7 @@ def _run_make_install(root: Path) -> int:
 
 def _run_pip_editable(root: Path) -> int:
     cmd = [sys.executable, "-m", "pip", "install", "-e", str(root)]
-    return subprocess.run(cmd, cwd=str(root), check=False).returncode
+    return subprocess.run(cmd, check=False).returncode
 
 
 @click.command("update")