Centralize slash_command routing for perf-improver, pr-fix, and test-improver (#353)

* Initial plan

* Add centralized slash command strategy to three workflows

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

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: dsyme <7204669+dsyme@users.noreply.github.com>
Co-authored-by: Peli de Halleux <pelikhan@users.noreply.github.com>

Author: 3 people

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