Fix task bugs and add search filter

[hariprasanth-aivar]

Changes

• Bug A fix: Status filtering now compares against the query parameter value instead of a string literal
• Bug B fix: Task completion now persists to disk via save_tasks()
• New feature: Case-insensitive q search parameter filters tasks by title and description, composes with status filter

Verified

• GET /tasks?status=open returns only open tasks
• GET /tasks?status=done returns only done tasks
• POST /tasks/{id}/complete persists and sets completed_at
• GET /tasks?q=launch matches case-insensitively
• GET /tasks?status=open&q=plan composes both filters correctly