Merge development: co-author gate, task tool, sessions

Brings in the merged #21, #23, and #24. Resolutions worth noting: The co-author attribution gate and the background path now share exec_run_command: a foreground command keeps the HEAD check and the amend, while a background task skips the gate, since its commits finish after the tool has already returned and there is nothing to amend from. The permission classify lists and the test module union both sides. Also fixes the Windows test job the same way #21 did: the capping test drove PowerShell through cmd /C with a quoted one-liner, which cmd mangles. It now uses a quote-free cmd-native for /L loop, with per-platform line markers in the assertions.