Cherry-pick PR #12179 (appmod fixes) to develop.next

[haoozhang]

Summary

• Cherry-pick merge commit from #12179 (merged to develop) into develop.next.
• Fixes Copilot Modernization branding, CVE fix actions, and menu hover issues in the appmod plugin.

Changes included

• Rebranding: install prompts now say "GitHub Copilot Modernization" instead of "App Modernization"
• CVE fix actions now use the designated custom agent; restored missing "Fix the vulnerable with GitHub Copilot" action in Problems view context menu
• Fixed "Install GitHub Copilot Modernization" suffix being repeatedly appended on menu hover

Test plan

• Verify appmod menu items show correct branding
• Verify CVE fix action works with custom agent
• Verify menu hover does not duplicate suffix

🤖 Generated with Claude Code

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