skill: create add-new-model and code-review (#26042)

* skill: add new model

* add common pitfalls

* add code review skill

* nits

* add codeowners

* add security review section

* mention about skills in agents.md

* add design review

* exclude ggml-gh-bot

* Apply suggestions from code review

Co-authored-by: Piotr Wilkin (ilintar) <piotr.wilkin@syndatis.com>
Co-authored-by: Xuan-Son Nguyen <thichthat@gmail.com>

* conversion-time weight modification

* nits

---------

Co-authored-by: Piotr Wilkin (ilintar) <piotr.wilkin@syndatis.com>

docs/development/HOWTO-add-model.md

@@ -45,6 +45,8 @@ class MyModel(MmprojModel):
 
 Add an enum entry in `MODEL_ARCH`, the model human friendly name in `MODEL_ARCH_NAMES` and the GGUF tensor names in `MODEL_TENSORS`.
 
+NOTE: Pick the GGUF arch string (and the matching `src/models/<name>.cpp` filename, see section 3) carefully up front, following existing naming conventions. Once GGUF files are published under a given arch string, renaming it later breaks the community's existing files, so this is not something to leave for cleanup in a follow-up PR.
+
 Example for `falcon` model:
 ```python
     MODEL_ARCH.FALCON: [
@@ -101,6 +103,7 @@ The model params and tensors layout must be defined in `llama.cpp` source files:
     - You may also need to update `LLM_KV_NAMES`, `LLM_TENSOR_NAMES` and `LLM_TENSOR_INFOS`
 3. Add any non-standard metadata loading in the `llama_model_loader` constructor in `src/llama-model-loader.cpp`.
 4. If the model has a RoPE operation, add a case for the architecture in `llama_model_rope_type` function in `src/llama-model.cpp`.
+5. Check for other places that switch/iterate over every `llm_arch` value, e.g. `src/llama-model-saver.cpp` and any mandatory-hparam lists (such as which archs require MoE metadata). Grep for `LLM_ARCH_` usages to find them. Missing one of these is a common cause of CI test failures (e.g. `test-llama-archs`) after adding a new arch.
 
 NOTE: The dimensions in `ggml` are typically in the reverse order of the `pytorch` dimensions.
 
@@ -133,6 +136,14 @@ Note:
 
 ## Tips and tricks
 
+### Prefer conversion-time tensor modifications over graph-time ones
+
+If the model contains constant modifications of tensors in the graph (for example, `norm(1 + weight)`) or performs tensor permutations/chunking, perform the modifications during conversion rather than in the graph code. This keeps the inference graph simpler and avoids extra runtime ops.
+
+Examples:
+- Gemma 3 folds the `1 +` of its `norm(1 + weight)` normalization into the weights at conversion time, so the graph just does a plain RMS norm.
+- Qwen3-Next applies its tensor permutation during conversion (in `modify_tensors`), so the graph can consume the already-permuted weights directly.
+
 ### Working with ggml_rope_ext
 
 PyTorch implementations usually prefer explicitly calculating `freq_cis`/`sin`/`cos` components. However, in llama.cpp, most RoPE operations can be handled via `ggml_rope_ext`, which does not require a sin/cos matrix. This saves memory while allowing the GGML RoPE kernel to be fused with other ops.