fix(ci): add repository check to prevent workflows from running on forks

Add if: github.repository == 'google/adk-python' guard to workflows
that lack this protection: v2-sync, issue-monitor, and copybara-pr-handler.
These workflows use secrets unavailable on forks, causing daily failures.

Author: Wu-Jiayang

.github/workflows/copybara-pr-handler.yml

@@ -17,6 +17,7 @@ on:
 
 jobs:
   close-imported-pr:
+    if: github.repository == 'google/adk-python'
     runs-on: ubuntu-latest
     permissions:
       pull-requests: write

.github/workflows/issue-monitor.yml

@@ -30,6 +30,7 @@ on:
 
 jobs:
   sweep-spam:
+    if: github.repository == 'google/adk-python'
     runs-on: ubuntu-latest
     timeout-minutes: 120
     permissions:

.github/workflows/v2-sync.yml

@@ -13,6 +13,7 @@ permissions:
 
 jobs:
   sync:
+    if: github.repository == 'google/adk-python'
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6