Add paths=True support to nbrg/nbrga and extract shared _paths_reduce helper

[ncoop57]

Summary

• nbrg/nbrga now support paths=True to return unique matched notebook paths as PathResults, matching the existing rg API. paths is mutually exclusive with count (asserted at call time).
• Extracted _paths_reduce from the inline dedup logic in rg() and _rg_post() so both rg and nbrg share one implementation for path reduction, capping, and timeout flags.
• rg() paths-with-timeout now goes through the unified _rg_post → _paths_reduce path instead of a separate code branch, fixing consistent stop_reason reporting.
• Updated README and skill docs to reflect nbrg's new paths/count parity with rg.