feat: diecut init — minimal template scaffolding for authors

[raiderrobert]

Problem

Template authoring has no easy entry point. Authors either hand-write diecut.toml from scratch reading docs, or reach for extract/distill (#117, #123, #130) which are powerful but overkill for someone just starting. Previous attempts have focused on reverse-engineering existing projects; this ticket is the simpler complement.

The explicit goal: start simpler, not smarter.

Proposed feature

diecut init <name> creates a minimal working template skeleton:

<name>/
├── diecut.toml           # minimal manifest, one variable
└── {{ project_name }}/
    └── README.md         # hello-world content using the variable

Behavior:

• Offline, <1s to create
• The generated skeleton renders successfully via diecut new <name> with zero edits
• Comments inside diecut.toml point to docs for extending (hooks, conditionals, computed vars)

Non-goals

• No reverse-engineering (that's extract / distill)
• No interactive prompts at init time — keep it minimal
• No multi-template repo scaffolding in v1
• No install-snippet generation (separate ticket for diecut share)

Open questions

• diecut init <name> (create new dir) vs. diecut init . (scaffold in place)?
• Include a commented-out hook stub, or omit entirely for "start simple"?

Acceptance criteria

• init subcommand wired up
• Generates the minimal skeleton
• Generated skeleton renders with diecut new <name> in tests
• Docs include a short authoring walkthrough