Clear pending subagent files on adopt; use them for deletions

Two review findings on #2646:

• session adopt reset the checkpoint window but kept the source's pending subagent files, whose baselines describe the source worktree. The target's next prompt attribution would measure from them.
• computeAgentDeletions still read the stale snapshot for a pending file, so a subagent deletion committed before the next snapshot counted as neither agent nor user work. It now uses the observed content too.