fix(ci): use pull_request_target and add write permissions for reviewer assignment

Fork PRs lack write access under `pull_request` trigger, causing reviewer
assignment to fail silently. Switch to `pull_request_target` and explicitly
grant `pull-requests: write` permission.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bladehan1

.github/workflows/pr-reviewer.yml

@@ -1,15 +1,17 @@
 name: Auto Assign Reviewers
 
 on:
-  pull_request:
+  pull_request_target:
     branches: [ 'develop', 'release_**' ]
     types: [ opened, edited, reopened ]
 
 jobs:
   assign-reviewers:
     name: Assign Reviewers by Scope
-    if: github.event_name == 'pull_request'
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
 
     steps:
       - name: Assign reviewers based on PR title scope