The 'persist /new boundary' change (#5730) made continueRecent honor
fresh /new breadcrumbs: a materialized fresh session is resumed instead
of the pre-/new transcript. The legacy migration test still asserted the
old resume-pre-/new behavior, and ran against the real terminal id
(WT_SESSION/TMUX_PANE), so it failed whenever those env vars were set.
- Scope terminal breadcrumbs to a deterministic test id and agent dir
- Update the /new contract test: materialize the fresh session (append
user+assistant message, flush), then assert continueRecent resumes it