docs: add non-custodial MCP server guide for RETURN_BYTES mode

#353 (predicates epic) must be fully complete before starting this task. Ideally land after T6 (#365) so the guide reflects the final envelope format.

JS reference: [PR #1009](https://github.com/hashgraph/hedera-agent-kit-js/pull/1009)
Python tracking: [Issue #315](https://github.com/hashgraph/hedera-agent-kit-py/issues/315)

## Scope

- Add `docs/MCP_NONCUSTODIAL.md` — walkthrough of the `RETURN_BYTES` + MCP pattern for non-custodial signing
- Document the full `ReturnBytesToolResponse` envelope fields
- Cross-link from the main README MCP section

First confirm predicates epic #353 is complete and review T6 #365 for the final envelope format. Add docs/MCP_NONCUSTODIAL.md covering the RETURN_BYTES and MCP non-custodial signing pattern and all ReturnBytesToolResponse fields, then cross-link it from the README MCP section; use JS PR #1009 and Python issue #315 as references.

Written by the indexing model from the issue text.