feat: Replace todo.txt with persistent brainfile-backed task files

UpdateTodoList now writes to structured task files in .cecli/tasks/board/
instead of a flat .cecli/todo.txt. The agent and user experience is
identical — same tool, same prompt, same ✓/○/→ display — but todos now
persist across sessions, maintain history in logs/, and support power-user
access via /task commands.

Key changes:
- UpdateTodoList always routes through CecliTaskStore (single write path)
- Session tasks auto-create on first UpdateTodoList call
- get_todo_list() auto-resumes incomplete tasks from previous sessions
- Task files never leak into agent context (only <context> block visible)
- Removed 7 board-only LLM tools (TaskCreate, TaskList, etc.)
- Added /task new, removed /task promote
- Session save/restore preserves active_task_id
- 45 tests passing

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 1broseidon

cecli/brainfile/__init__.py

@@ -0,0 +1,3 @@
+from cecli.brainfile.store import CecliTaskStore
+
+__all__ = ["CecliTaskStore"]