refactor(graph): rename rules to modules

Summary

• Renames the graph extension surface from rules to modules across Rust frontend APIs, manifest loading, and action digest terminology.
• Switches the Starlark declaration helper from rule(...) to target_kind(...), and renames discovery from once query rules / once_list_rules to target kind surfaces.
• Updates generated CLI/MCP references, guide docs, RFC language, and shell specs for [modules] and target_kind(...).

Testing

• mise exec -- cargo fmt --all -- --check
• mise exec -- cargo test -p once-frontend -p once-cli
• mise exec -- cargo build --release -p once-cli --bin once
• mise exec -- shellspec spec/graph_edit_spec.sh spec/graph_action_spec.sh
• mise exec -- shellspec spec/apple_spec.sh
• git diff --check

No GitHub comments yet.