Allow adding todos to past days
Remove the past-date guard that prevented adding todos to days before today.
Three code paths enforced this restriction (CLI, tool function, UI template)
and one test asserted it — all updated.
Changes:
- apps/todos/call.py: Remove past-date rejection block, keep format validation
- apps/todos/tools.py: Remove past-date check, update docstring
- apps/todos/app_bar.html: Remove is_future_or_today guard, enable input for all days
- apps/todos/tests/test_call.py: Convert test_add_past_date_rejected to test_add_past_date_allowed
- apps/todos/muse/todos/SKILL.md: Remove "must be today or in the future" from --day docs
- apps/todos/tests/test_tools.py: Update stale comment about date validation
Note: make ci has one pre-existing failure in test_cortex_client.py unrelated to this change.