feat: multi-template catalog generation (diecut share at repo root)

[raiderrobert]

Problem

Some template repos host multiple templates in subdirectories — e.g. raiderrobert/templates with rust-cli, godot-tag-game, claude-code-plugin. Authors currently hand-write a catalog README with install instructions for each. The diecut share feature handles a single template; this ticket extends it to emit a catalog when run at a repo root.

Proposed feature

When diecut share runs at the root of a repo with no diecut.toml directly in cwd but one or more subdirectories containing diecut.toml, emit a catalog:

# Templates

| Template | Description | Install |
|---|---|---|
| rust-cli | A command-line tool | `diecut new gh:user/templates/rust-cli` |
| godot-tag-game | A game built with Godot | `diecut new gh:user/templates/godot-tag-game` |

• Template name = subdirectory name
• Description = new optional description field in diecut.toml (or first line of template's own README as fallback)
• Install command = what single-template diecut share would emit for each subdirectory

Dependencies

• diecut share (single-template mode) must land first — this is a mode extension of the same command.

Open questions

• Description source: new diecut.toml field, parsed README first line, or both?
• Write target: repo root README.md (with fence markers) or dedicated CATALOG.md?

Acceptance criteria

• Catalog mode triggers automatically at repo root with no diecut.toml present but subdirectories containing one
• Scans subdirectories, emits markdown table
• Description field support
• Tests with a multi-template fixture repo
• Docs