fix(checklist): clarify file handling behavior for append vs create (#1556)

* fix(checklist): clarify file handling behavior for append vs create

Resolves contradictory instructions in checklist.md lines 97-99 where the
template stated both 'append to existing file' and 'creates a NEW file'.

Changes:
- Clarified that if file doesn't exist, create new with CHK001
- If file exists, append new items continuing from last CHK ID
- Emphasized preservation of existing content (never delete/replace)

Co-authored-by: Augment Agent <noreply@augmentcode.com>

* fix(checklist): align report language with append behavior

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Co-authored-by: Augment Agent <noreply@augmentcode.com>