Announce only commits that hold a candidate's work; keep the snapshot allocation-free

The unlinked-commit notice fired on any human commit made while sessions lived in two or more other worktrees, even idle ones holding nothing this commit stages. It now names only candidates with new content in the commit.

The pending-content snapshot built two maps on every mutation, PostToolUse included. It now keeps the FilesTouched slice header and a digest of the task records, allocating only when FilesTouched was replaced by a same-length list.