fix(desktop): surface background-session clarify prompts instead of hanging

clarify.request is a one-shot blocking event: the gateway turn blocks on
clarify.respond. The desktop handler dropped it for any non-focused session
(`if (!isActiveEvent) return`) and stored at most one request in a single
global atom, so a background session that asked a clarifying question hung
forever and re-focusing it could never recover (the event was already gone).

- store/clarify.ts: key pending requests by runtime session id; expose the
  active session's request via a focus-scoped computed view (ClarifyTool is
  unchanged). clearClarifyRequest takes an optional session id for targeted
  clears, with a request-id fallback.
- use-message-stream.ts: park every session's clarify (drop the isActiveEvent
  early return); toast when one lands for a background session since the row
  otherwise just keeps spinning like normal work.
- clarify-tool.tsx: clear by session id so answering one chat can't wipe
  another's pending request.
- store/clarify.test.ts: concurrent independence, focus-scoped view,
  targeted/stale/fallback clears.

apps/desktop/src/components/assistant-ui/clarify-tool.tsx

@@ -103,7 +103,7 @@ function ClarifyToolPending({ args }: ToolCallMessagePartProps) {
           answer
         })
         triggerHaptic('submit')
-        clearClarifyRequest(matchingRequest.requestId)
+        clearClarifyRequest(matchingRequest.requestId, matchingRequest.sessionId)
         // The matching tool.complete will land shortly after, swapping this
         // panel for the ToolFallback view above.
       } catch (error) {