Recover a session's context from the CLI's own file

A restarted server has been told nothing, so a Claude session that has not
taken a turn since reported its context as unknown -- which was true, and
useless, since the CLI had written the figure down at the time and it was
sitting in the session's file the whole while.

It now reads it from there at load, over the session's transport, in the
background: the same three input fields the import list already reads, so
it is a measurement rather than a guess. Only when nothing else has
answered, and only for a provider that keeps such a file.

A clear needs no special case even though it makes the last usage in a
file stale, because clearing gives the CLI a new session id -- so the
lookup lands on a file with no usage yet and answers unknown, which is
what it is.

PLAN.md

@@ -193,6 +193,12 @@ it touches the transcript or the phone:
   and the phone folds with the same one (2026-08-30: this replaced a running
   spend total, which could only climb and so kept reporting a context a
   compaction or a clear had already taken away).
+  A session the server has no measurement of asks the CLI's own file
+  instead of waiting for a turn — `import::context_of`, the same three
+  fields the import list reads, in the background at load so a start never
+  waits on an ssh. A clear needs no special case: it gives the CLI a new
+  session id, so the lookup lands on a file with no usage in it and
+  answers "unknown", which is true.
 - `Error { message }`.
 
 Every event is appended to the session's transcript file with a sequence