Add op_model_target

[Bouteillepleine]

  target_model:
    description: 'Optional exact model filter, e.g. OP15. Leave empty to build all models matching op_model.'
    required: false
    type: string
    default: ''  

Added optional target_model support to the workflow, allowing builds to be filtered to a specific device model using an exact .model match. The filter is applied during matrix generation after the selected op_model group is resolved. Build plan summaries, per-job summaries, release notes, and final release summaries now display the selected target model

https://github.com/Bouteillepleine/OnePlus_Kernel_SUSFS/actions/runs/25819074352

https://github.com/Bouteillepleine/OnePlus_Kernel_SUSFS/actions/runs/25819023993

[fatalcoder524]

I would suggest a drop-down with all options and first option being "ALL" which is also default option.

There is already a implementation similar to this here: https://github.com/fatalcoder524/OnePlus-Remote-Action-Build/blob/51d90bca2fe8df8fd111fd25c531860db70e3a9d/.github/workflows/build-kernel-release.yml#L14