macp-runtime 0.7.4

MACP reference runtime: a coordination kernel and gRPC server enforcing session boundaries, message validation, append-only history, modes, and governance policy.
Documentation

## Summary

Describe the changes introduced by this pull request.

## Type of Change

- [ ] New feature
- [ ] Bug fix
- [ ] Refactoring
- [ ] New mode implementation
- [ ] Proto schema change
- [ ] Documentation improvement
- [ ] CI / tooling change

## Checklist

- [ ] `cargo test` passes
- [ ] `cargo clippy` has no warnings
- [ ] `cargo fmt` has been run
- [ ] Protobuf definitions compile successfully
- [ ] New modes implement the `Mode` trait correctly (if applicable)
- [ ] Error variants added to `MacpError` (if applicable)
- [ ] This change preserves MACP Core invariants