Add todo list point lookup

[huangruiteng]

Summary

• allow loopx todo list --todo-id <id> as a read-only point lookup on the existing todo list surface
• return optional point-lookup fields only when --todo-id is provided, keeping default list output unchanged
• include compact relation fields from the matched todo without introducing a new runtime contract

Validation

• python3 examples/todo-cli-smoke.py
• python3 -m py_compile loopx/todos.py loopx/cli_commands/todo.py
• git diff --check

Notes

• The smoke covers default list output, user-gate lookup, agent-todo lookup, and missing-id lookup.
• This is a narrow CLI behavior slice, so it should be reviewed before merging.