🤖 Add defensive checks for init-output and message processing

Fixes two issues found after PR #228:

1. **Init-output undefined line crash**: Added null check for data.line
   before calling trimEnd(). Prevents crash when init-output events
   arrive with missing line data during replay or out-of-order scenarios.

2. **Message processing condition tightened**: Added 'role' field check
   to ensure only CmuxMessages are processed in the caught-up branch.
   This makes the condition symmetric with the buffering branch and
   ensures we don't accidentally process malformed messages.

Both changes follow the defensive programming pattern established in
PR #228 - gracefully handle edge cases during replay rather than crash.

_Generated with `cmux`_

Author: ammar-agent

src/stores/WorkspaceStore.ts

@@ -964,12 +964,15 @@ export class WorkspaceStore {
       const historicalMsgs = this.historicalMessages.get(workspaceId) ?? [];
       historicalMsgs.push(data);
       this.historicalMessages.set(workspaceId, historicalMsgs);
-    } else if (isCaughtUp) {
+    } else if (isCaughtUp && "role" in data) {
       // Process live events immediately (after history loaded)
+      // Check for role field to ensure this is a CmuxMessage
       aggregator.handleMessage(data);
       this.states.bump(workspaceId);
       this.checkAndBumpRecencyIfChanged();
     }
+    // Note: Messages not matching above conditions are silently ignored
+    // This is expected for malformed messages or events handled elsewhere
     // Note: Init events and stream events are handled by isStreamEvent() buffering above
   }
 }

src/utils/messages/StreamingMessageAggregator.ts

@@ -512,6 +512,7 @@ export class StreamingMessageAggregator {
 
     if (isInitOutput(data)) {
       if (!this.initState) return; // Defensive: shouldn't happen but handle gracefully
+      if (!data.line) return; // Defensive: skip events with missing line data
       const line = data.isError ? `ERROR: ${data.line}` : data.line;
       this.initState.lines.push(line.trimEnd());
       this.invalidateCache();