feat(coach): keep looking after the round cap

When the tool loop forces an answer at its round cap, the reply leads
its chips with "Keep looking". Tapping it runs a normal turn, under the
budget gate as ever, with two server-written context lines: the question
that was left unfinished (at most 1000 characters) and what the forced
turn already read, each table by the name show_result takes, so the
continued turn shows those again instead of fetching them twice.

An answer is continued at most once. The continued reply records the
message it continues in metricSource.continuationOf and offers no
further "Keep looking", even when it is forced again; a chip on a reply
that is itself a continuation is refused.

docs/api/openapi.yaml

@@ -50399,6 +50399,11 @@ components:
           description: v1.39.4 — the answer was forced at the round cap while the Coach still wanted to read. Absent otherwise.
           type: boolean
           const: true
+        continuationOf:
+          description: "v1.39.4 — this reply continues an answer that was forced at the round cap (the `continue` chip): the id of
+            that earlier assistant message. A continuation offers no further `continue` chip. Absent otherwise."
+          type: string
+          maxLength: 64
       required:
         - windows
         - metrics