fix(strategy): count each checkpoint's tokens once

CheckpointTranscriptStart chose both which part of the transcript a checkpoint shows and where its token count starts. Carry-forward after a partial commit and session adopt reset it to 0 so the next checkpoint shows the whole conversation, which also made that checkpoint count every token from the start of the session again. Session attach always counted from 0 and never recorded how far it got, and the turn-end advance after a mid-turn commit skipped that turn's remaining tokens entirely.

Add a separate offset, TokenTranscriptStart, that advances only when a condensation stores tokens. Token counting in condensation, the live transcript path, review sessions and attach reads it through TokenStart(). Attach records the transcript end afterwards and resets the pending window. Adopt keeps the source's token offset, pending total and subagent baseline, because the retired source never condenses again. State written by an older CLI starts from CheckpointTranscriptStart, as before.