fix(codex): resolve the current turn from the rollout when the Stop payload carries no turn_id (#1534)

Codex added turn_id to StopCommandInput in rust-v0.117.0 (0.116.0-alpha.12),
after the hooks engine shipped in rust-v0.114.0, so stable rust-v0.114.0,
v0.115.0 and v0.116.0 fire the Stop hook with no turn_id at all. #1169 made
resolveCodexStopPlan fail closed on that payload, which silently disabled plan
review on those versions.

When the payload omits the field, derive the turn from the rollout instead: the
id of the last id-carrying turn_context/task_started marker, then the same
first-marker-anchored scan, so mid-turn compaction safety and per-turn plan
filtering are unchanged. A rollout with no turn markers still fails closed, and
so does a payload that carries turn_id as a blank string (truncated or foreign,
refused before the file is read).

Using the fallback writes one line to stderr, unconditionally — stdout is the
hook's JSON decision channel, and Codex only reads a Stop hook's stderr on exit
code 2, which this hook never uses.

Docs: minimum-Codex-version table in the Codex troubleshooting guide and a
turn-resolution section in AGENTS.md.