Add `gt molecule` subcommands for managing workflow templates:
- list: Show all molecules (type=molecule issues)
- show: Display molecule with parsed step structure
- parse: Validate molecule and show parsed details
- instantiate: Create child beads from molecule template
- instances: Show all instantiations of a molecule
Also add `--molecule` flag to `gt spawn` for molecule-based workflows.
When specified, the molecule is instantiated on the parent issue first,
then the polecat is spawned on the first ready step.
🤝 Co-authored-by: Claude <noreply@anthropic.com>
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