Add promote_running to CompanionManager: scans STARTING companions and moves
any that have stayed alive at least their startsecs window to RUNNING, logging
the pid and returning the promoted ones. Companions that die inside the window
are left to reaping.
Add tests for promotion after the window, too-early no-op, and non-STARTING.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>