Keep shadow-branch steps pinning a session whose home was removed

The removed-home re-home skipped the whole pending-content guard, so a session with uncondensed shadow-branch steps could move: re-homing rewrites BaseCommit and WorktreeID, the key the shadow branch is named by, without moving the ref, and every later lookup would miss those steps. A removed home now releases a session only for files and task records; steps still hold it, and that case is logged. The common flow (committed, merged, then removed) has no steps left.