Script-Backed Fallback for Prompt-Layer File Checks

Problem

/vt-v-start SKILL.md instructed Claude to check for .cwp-checkpoint.yaml before running start_scanner.py. If the checkpoint existed, Claude was supposed to show a P0 resume banner.

The script had no awareness of the checkpoint. This meant: - The check depended entirely on Claude following the SKILL.md instruction correctly - Any variation in prompt execution caused checkpoint detection to silently fail - Users would see no P0 banner and miss in-progress CWP resume cues

Solution

Add scan_checkpoint(repo_root) to start_scanner.py:

def scan_checkpoint(repo_root: Path) -> Optional[dict]:
    """Read .cwp-checkpoint.yaml if present — enables P0 resume detection in the script."""
    checkpoint_file = repo_root / '.cwp-checkpoint.yaml'
    if not checkpoint_file.exists():
        return None
    try:
        data = yaml.safe_load(checkpoint_file.read_text(encoding='utf-8'))
        return data if isinstance(data, dict) else None
    except (yaml.YAMLError, OSError):
        return None

Wire through compute_priority() as a new optional parameter:

def compute_priority(..., checkpoint: Optional[dict] = None) -> dict:
    # P0: Resume checkpoint (highest — before git branch P0)
    if checkpoint:
        actions.append({'priority': 'P0', 'label': 'Resume ...', ...})
    # P0: In-flight git branch
    ...

The checkpoint banner also renders in format_markdown() before the status table.

Pattern: Script-Backed Prompt Fallback

For any "if file exists, show as high priority" instruction in SKILL.md prompts:

1. Back it with a script function that reads the file and returns structured data
2. Thread the data through the main processing function (priority, formatting)
3. Keep the SKILL.md instruction for context and documentation — but it becomes a user-facing explanation, not the sole enforcement mechanism

This ensures the behavior is reproducible even when prompt execution is imperfect.

Prevention

Whenever a SKILL.md says "before running the script, check for X" — that check should also exist inside the script. The rule of thumb:

If losing the prompt instruction would silently break a key UX behavior, the behavior must also be implemented in the script.