Detection: perch-config manifest existence implies detected

[Laoujin]

Summary

If a YAML manifest exists in the user's perch-config repo for an app, treat it as detected. The user has chosen to manage this app -- that's an implicit signal it's installed.

Code change:

• During detection, check if the app ID has a corresponding manifest in perch-config
• If manifest exists, mark as detected regardless of other detection methods
• This serves as the "manual override" for detection -- users add a manifest to declare they have the app

Test Plan

• App with manifest in perch-config but no package/detect match shows as Detected
• App with both manifest and package match still works correctly
• Removing manifest removes the detection override