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>

Author: zaheer15351

templates/commands/checklist.md

@@ -94,9 +94,10 @@ You **MUST** consider the user input before proceeding (if not empty).
    - Generate unique checklist filename:
      - Use short, descriptive name based on domain (e.g., `ux.md`, `api.md`, `security.md`)
      - Format: `[domain].md`
-     - If file exists, append to existing file
-   - Number items sequentially starting from CHK001
-   - Each `/speckit.checklist` run creates a NEW file (never overwrites existing checklists)
+   - File handling behavior:
+     - If file does NOT exist: Create new file and number items starting from CHK001
+     - If file exists: Append new items to existing file, continuing from the last CHK ID (e.g., if last item is CHK015, start new items at CHK016)
+   - Never delete or replace existing checklist content - always preserve and append
 
    **CORE PRINCIPLE - Test the Requirements, Not the Implementation**:
    Every checklist item MUST evaluate the REQUIREMENTS THEMSELVES for: