🤖 fix: bump usageStore when MuxMessage is stored (#696)

When compaction is interrupted, we don't receive usage metadata from the
interruption event itself. However, we do have historical usage data
from messages stored in the aggregator. Without bumping usageStore when
MuxMessages are received, the UI doesn't re-render to display this
historical usage, making it appear as if costs have reset.

This single-line addition ensures the usage store stays in sync with
message arrivals, so historical usage data is properly reflected in the
UI.

## Changes
- Added `this.usageStore.bump(workspaceId)` after live MuxMessage
storage in WorkspaceStore

## Testing
- Verified typecheck passes
- Change follows existing pattern from stream-end/stream-abort handlers

Relates to #695

_Generated with `mux`_

Author: ethanndickson

src/browser/stores/WorkspaceStore.ts

@@ -818,6 +818,7 @@ export class WorkspaceStore {
         // Process live events immediately (after history loaded)
         aggregator.handleMessage(data);
         this.states.bump(workspaceId);
+        this.usageStore.bump(workspaceId);
         this.checkAndBumpRecencyIfChanged();
       }
       return;