Load custom workspace rules from the root once.toml

You can now add custom Starlark rule files to your workspace from the root once.toml. Declare a [rules] section with paths to .star files, for example paths = [“rules/*.star”]. Once loads them alongside the built-in prelude, validates them, and makes the new target kinds available through once query target-kinds, once query schema, the edit command, the MCP tool list, and graph analysis. This lets teams and ecosystems extend Once with their own target kinds without modifying the core. Rule files are resolved relative to the project root, must stay inside the workspace, and cannot be placed under the .once state directory. Custom rule source is also included in action cache input digests, so changing a rule correctly invalidates cached build outputs.