refactor: refactor TODO handling to context messages

- Refactor TODOs handling: move logic to `context_messages` instead of using `template_opts`
- Improve todo message filtering by using more precise tags (`<todos>...</todos>`)
- Append TODOs after history messages
- Remove the TODOs section from `_task-guidelines.avanterules` template

Signed-off-by: doodleEsc <cinuor@gmail.com>