new: Add community plugin registry

[milesj]

Summary

• generate registry/data/community.json nightly from moonrepo/community-plugins
• resolve community tools automatically after configured and built-in plugins, using the shared HTTP-backed registry cache
• add settings.community-tools and PROTO_COMMUNITY_TOOLS opt-outs
• reuse registry instances through ProtoEnvironment::load_registry

Testing

• python3 -B -m unittest registry/tests/test_generate_community_registry.py
• cargo test -p proto_core --test config_test can_set_settings
• cargo test -p proto_core --test tool_loader_test locate_plugin
• cargo check -p proto_cli
• cargo clippy -p proto_cli
• cargo fmt --all --check