tests : add fusion baseline README and broaden fusion CI triggers (#28893)

* tests : add README for updating the per-backend fusion baselines

Assisted-by: pi:llama.cpp/Qwen3.8-27B

* ci : trigger fusion on changes to test-llama-archs.cpp and src/models

the dummy models and their architectures drive the fusion baselines, so a
change to either can alter the per-fusion counters and should re-run the
fusion job.

Assisted-by: pi:llama.cpp/Qwen3.8-27B

* tests : merge the fusion build commands in the README

assisted-by: pi:llama.cpp/Qwen3.8-27B

* pi : require explicit permission before posting PR/issue comments

assisted-by: pi:llama.cpp/Qwen3.8-27B

.github/workflows/fusion.yml

@@ -9,7 +9,9 @@ on:
       '.github/workflows/fusion.yml',
       'ggml/**',
       'tests/fusion/**',
-      'tests/test-fusion.cpp'
+      'tests/test-fusion.cpp',
+      'tests/test-llama-archs.cpp',
+      'src/models/**'
     ]
 
   pull_request:
@@ -18,7 +20,9 @@ on:
       '.github/workflows/fusion.yml',
       'ggml/**',
       'tests/fusion/**',
-      'tests/test-fusion.cpp'
+      'tests/test-fusion.cpp',
+      'tests/test-llama-archs.cpp',
+      'src/models/**'
     ]
 
 concurrency: