capture: answer the stop before summarising, and always leave a note

capture_stop held the IPC request open for the whole map reduce, up to
twenty minutes. A voice turn that says "хватит" waited for forty model
calls before Maven said anything. Stop now returns the transcript and the
summary runs on a goroutine in the daemon's WaitGroup, on the daemon
context so a client that hung up does not cancel the only readable record
of the meeting.

With no summary and save_transcript false, writeNotes wrote nothing at
all: an hour of meeting left a blob that prunes in seven days and no
trace in the note store. The transcript is written instead when the
summary is missing. That flag decides whether the verbatim record is kept
in addition to a summary, not whether the meeting is remembered.

The wire carries the session token now, and the contract comments say
what the code does: the summary is usually absent from the stop
response, and re running a stored blob is a manual job because no method
takes a blob id. The save_transcript comment says the cost is recall
corpus rather than disk.

Found in review of #73.

internal/ipc/client.go

@@ -500,9 +500,10 @@ func (c *Client) CaptureAppend(ctx context.Context, req CaptureAppendReq) (Captu
 	return r, nil
 }
 
-// CaptureStop ends the session. Slow — it transcribes and summarises the whole
-// recording — so pass a context with room. Set Discard to throw the recording
-// away instead.
+// CaptureStop ends the session. It transcribes the whole recording before
+// answering, so pass a context with room; the summary is written afterwards by
+// the daemon and is usually absent from the response. Set Discard to throw the
+// recording away instead. Token comes from CaptureStart.
 func (c *Client) CaptureStop(ctx context.Context, req CaptureStopReq) (CaptureStopResp, error) {
 	var r CaptureStopResp
 	if err := c.call(ctx, MethodCaptureStop, req, &r); err != nil {