Add centralized slash command strategy to three workflows

Co-authored-by: dsyme <7204669+dsyme@users.noreply.github.com>

Author: Copilot

workflows/perf-improver.md

@@ -15,6 +15,7 @@ on:
   workflow_dispatch:
   slash_command:
     name: perf-assist
+    strategy: centralized
   reaction: "eyes"
   permissions:
     pull-requests: read

workflows/pr-fix.md

@@ -9,6 +9,7 @@ description: |
 on:
   slash_command:
     name: pr-fix
+    strategy: centralized
   reaction: "eyes"
 
 permissions: read-all

workflows/test-improver.md

@@ -15,6 +15,7 @@ on:
   workflow_dispatch:
   slash_command:
     name: test-assist
+    strategy: centralized
   reaction: "eyes"
   permissions:
     pull-requests: read