Consolidate find_task_file() and parse_checklist_items() into src/task.rs

**TaskGuard ID:** backend-026  
**Task File:** [tasks/backend/backend-026.md](https://github.com/Guard8-ai/TaskGuard/blob/master/tasks/backend/backend-026.md)
**Source Branch:** master
**Hash:** 39dabb4f

## Description

[Why this task exists and what problem it solves]

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*Synced from TaskGuard*