Migrate CLN hold invoice plugin to Boltz hold

[0xjc65eth]

Summary

This migrates the CLN hold-invoice path from the archived daywalker90/holdinvoice plugin to BoltzExchange/hold.

Changes included:

• build and install the hold binary in the CLN Docker image
• update CLN config/traditional regtest scripts to use hold-grpc-port and the new plugin binary name
• fetch the current BoltzExchange/hold proto in scripts/generate_grpc.sh
• adapt api/lightning/cln.py to the new gRPC API:
  • Invoice for hold invoice creation
  • List for invoice lookup
  • Settle with the payment preimage
  • Cancel with the payment hash
  • UNPAID / ACCEPTED / PAID / CANCELLED state mapping
• keep HTLC expiry tracking by reading cltv_expiry from accepted invoice HTLCs

Closes #2317.

Testing

• python3 -m py_compile api/lightning/cln.py
• .venv/bin/ruff check api/lightning/cln.py
• generated the current Boltz hold.proto stubs with grpc_tools.protoc in /tmp and smoke-tested the Python API symbols used by this PR
• bash -n docker/cln/entrypoint.sh
• sh -n scripts/traditional/regtest-nodes
• git diff --check

I did not run a full CLN regtest/docker flow locally, so the main thing I would still want confirmed in review is the runtime plugin startup plus one accepted/settled hold invoice path.