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

---------

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

Author: 2 people

templates/commands/checklist.md

@@ -113,9 +113,10 @@ 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:
@@ -227,13 +228,13 @@ You **MUST** consider the user input before proceeding (if not empty).
 
 6. **Structure Reference**: Generate the checklist following the canonical template in `templates/checklist-template.md` for title, meta section, category headings, and ID formatting. If template is unavailable, use: H1 title, purpose/created meta lines, `##` category sections containing `- [ ] CHK### <requirement item>` lines with globally incrementing IDs starting at CHK001.
 
-7. **Report**: Output full path to created checklist, item count, and remind user that each run creates a new file. Summarize:
+7. **Report**: Output full path to checklist file, item count, and summarize whether the run created a new file or appended to an existing one. Summarize:
    - Focus areas selected
    - Depth level
    - Actor/timing
    - Any explicit user-specified must-have items incorporated
 
-**Important**: Each `/speckit.checklist` command invocation creates a checklist file using short, descriptive names unless file already exists. This allows:
+**Important**: Each `/speckit.checklist` command invocation uses a short, descriptive checklist filename and either creates a new file or appends to an existing one. This allows:
 
 - Multiple checklists of different types (e.g., `ux.md`, `test.md`, `security.md`)
 - Simple, memorable filenames that indicate checklist purpose