feat: Support workspace rule paths

Summary

• Add root-level [rules] paths support for checked-in Starlark rule files.
• Thread workspace rule schemas through query, MCP, edit, graph loading, and rule analysis.
• Include rule source in declared action cache inputs and document the new configuration.

Testing

• mise exec -- cargo test -p once-frontend
• mise exec -- cargo test -p once-cli
• mise exec -- cargo test --workspace
• mise exec -- cargo clippy --workspace --all-targets -- -D warnings
• mise exec -- cargo fmt --all -- --check
• git diff --check

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