fix: Resolve 11 errcheck linter violations to unblock CI (bd-91)

Fixed all unchecked error returns in production code:
- os.Remove() calls in cleanup paths
- cmd.Wait() in goroutines
- fmt.Fprintf() writes
- Type assertions with proper ok checks

Reduces linter issues from 99 to 88. CI should now pass linting.

internal/rpc/server.go

@@ -2159,8 +2159,8 @@ func (s *Server) handleExport(req *Request) Response {
 	}
 	tempPath := tempFile.Name()
 	defer func() {
-		tempFile.Close()
-		os.Remove(tempPath)
+		_ = tempFile.Close()
+		_ = os.Remove(tempPath)
 	}()
 
 	// Write JSONL