docs: update CLI documentation to use username instead of user-id

CLI Changes:
- Update user.get command to accept username parameter instead of user-id
- Update user.update command signature in documentation
- Update user.delete command signature in documentation

Implementation Consistency:
- Align README.md CLI examples with app/cli/commands/user.py implementation
- Clarify username-based user identification across all user management commands
- Update test expectations to match username-based interface

Documentation:
- README.md: Replace <user-id> with <username> in all user command examples
- Ensure consistency across user create, get, update, delete commands

tests/test_cli.py

@@ -829,14 +829,14 @@ def test_cli_user_update_and_delete_flow() -> None:
 
     update_result = runner.invoke(
         runner.app,
-        ["user", "update", user.id, "--username", "alice-updated", "--force"],
+        ["user", "update", "alice", "--username", "alice-updated", "--force"],
     )
-    delete_result = runner.invoke(runner.app, ["user", "delete", user.id, "--force"])
+    delete_result = runner.invoke(runner.app, ["user", "delete", "alice-updated", "--force"])
 
     assert update_result.exit_code == 0
     assert "alice-updated" in update_result.stdout
     assert delete_result.exit_code == 0
-    assert f"Deleted user: {user.id}" in delete_result.stdout
+    assert "Deleted user: alice-updated" in delete_result.stdout
 
 def test_cli_comment_add_and_list() -> None:
     repo = FakeRepo(_portfolio())