Credit background subagent edits to the agent, not the user

A Claude Code background subagent finishes after the parent's turn ended. Its completion writes no shadow snapshot, and the task-notification starts the next turn in the same instant. Turn-start prompt attribution then diffs the worktree against a snapshot that lacks the subagent's edits and counts every line as user work, so the checkpoint records agent_lines: 0.

Track files a subagent wrote since the last snapshot in PendingSubagentFiles. Task completion adds them; turn start adds files from still-running subagents' transcripts, covering the notification turn that wins the race with SubagentStop. Turn-start attribution skips them, turn end snapshots them, and SaveStep clears them, including on a skipped step whose tree already holds them. A commit before the next snapshot uses their committed content in place of the stale snapshot, and condensation drops the committed ones.