Collection detection for nested template patterns

[raiderrobert]

Summary

When diecut extract encounters a directory like skills/ that contains multiple sibling subdirectories with similar structures, it should detect this as a "collection" and offer to generate a nested diecut.toml for that subdirectory pattern.

This would enable diecut new to scaffold a new item within an existing project — e.g., adding a new skill to a skills package without re-scaffolding the whole project.

Example

Given a source project with:

skills/
  convention-mining/       → SKILL.md, references/*.md
  iterating-skill-audit/   → SKILL.md, review-focus.md
  auditing-convention-skills/ → SKILL.md, references/*.md
  writing-skills/          → SKILL.md, craft.md, testing.md, ...
  reviewing-abstraction-boundaries/ → SKILL.md

extract would detect that all 5 subdirectories share SKILL.md (5/5 = high confidence), some share a references/ subdir (2/5 = lower confidence), and offer to:

1. Generate a sub-template with its own diecut.toml for the common pattern
2. Mine common content structure across the shared files (e.g., common markdown headings in SKILL.md)
3. Produce a stub example that captures the pattern

Design considerations

• Structural similarity (file tree shape) is cheap and answers "is this a collection?"
• Content similarity (within common files) is more expensive and answers "what does the stub look like?"
• Confidence thresholds should differ for these two layers
• Interactive confirmation: show detected collections and let users confirm/adjust
• The generated sub-template should be a full diecut.toml that works with diecut new independently