gnd: Implement --from-subgraph for init command

Add support for creating a subgraph scaffold from an existing deployed
subgraph. This fetches the manifest and schema from IPFS, validates the
network matches, finds immutable entities, and generates a scaffold with
grafting instructions.

Changes:
- Add IPFS cat/fetch_manifest/fetch_schema methods to IpfsClient
- Add manifest parsing utilities (extract_schema_cid, extract_network,
  get_min_start_block)
- Add GraphQL schema parsing to find immutable entities
- Implement full init_from_subgraph() async function
- Add 11 unit tests for parsing functions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: lutter