fix(codex): keep Stop plan review in the current turn (#1169)

* fix(codex): keep Stop plans in the current turn

* fix(codex): anchor the Stop turn on its first rollout marker

Mid-turn compaction re-emits a `turn_context` line carrying the same turn
id as the in-flight turn (Codex `Session::replace_compacted_history`), so
locating the turn by its LAST matching marker started the scan after the
compaction point and lost a plan the turn had already produced. Anchor on
the FIRST marker instead, covered by fixtures that place a plan item and an
assistant <proposed_plan> ahead of a `compacted` line.

Also on the Stop path:

- resolveCodexStopPlan answers "which plan" and "why none" from a single
  parse; getCodexStopSkipReason read the whole rollout and then
  getLatestCodexPlan read it again. The turn-id check still runs before the
  file is touched, so a Stop payload without one still loads no content.
- Drop the `!existsSync(rolloutPath)` re-check (the path already came from
  a `.find(existsSync)`) and the turn-id guards downstream of the gate that
  can no longer be false.
- Drop the unused findCodexRolloutByThreadId import.
- Fold the single-consumer plan-normalization module back into
  codex-session.ts.
- Refresh the two comments that still described the old
  degrade-to-last-turn-in-file fallback, which no longer exists.

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Co-authored-by: Michael Ramos <mdramos8@gmail.com>