Skip agent-stop fallback snapshots with no human baseline

A turn end with no detected change takes a fallback snapshot. After an agent commits mid-turn, condensation deletes the shadow branch and clears the step count and pending prompt diff, so that snapshot has no human diff for its window and can claim nothing as agent work. With human dirt in the worktree it still differed from HEAD, and was written: a shadow branch with no agent work survived the commit (E2E TestDirtyWorkingTree and TestMidTurnCommit_DifferentFilesThanPreviousTurn on main).