fix(start-work): always hand off to atlas regardless of current session agent (#3155)

/start-work is the "execute this plan with atlas" command.
Previously, if the user's current session agent was sisyphus (or any
non-prometheus/non-atlas agent), the hook would keep that agent instead
of switching to atlas. This caused sisyphus to execute the plan.

Fix: unconditionally select atlas (when registered) or sisyphus (fallback).
The current session agent is irrelevant to /start-work's intent.

Adds regression test: running /start-work from a sisyphus session must
always hand off to atlas when atlas is available.