* fix(cron): clear agentTurn thinking override when patched with null
Cron agentTurn patches could clear model/fallbacks/toolsAllow overrides by
sending an explicit null, but thinking had no clear path: the patch schema and
normalizer dropped thinking:null before it reached the merge logic, and the
payload merge only handled string values. Blanking the Thinking/Effort field in
the Cron Control UI therefore silently preserved the old value.
Add thinking:null support across the patch schema, exported type, normalizer,
and payload merge (mirroring model). The Control UI now sends an explicit clear
for model/thinking when an edited job blanks a previously stored override, and
the CLI gains --clear-thinking for parity with --clear-model.
* docs(cron): document --clear-thinking beside sibling clear flags
ba3f68030b