chore!: remove model mgmt from CLI for Hugging Face CLI (#3700)

This change removes the `llama model` and `llama download` subcommands
from the CLI, replacing them with recommendations to use the Hugging
Face CLI instead.

Rationale for this change:
- The model management functionality was largely duplicating what
Hugging Face CLI already provides, leading to unnecessary maintenance
overhead (except the download source from Meta?)
- Maintaining our own implementation required fixing bugs and keeping up
with changes in model repositories and download mechanisms
- The Hugging Face CLI is more mature, widely adopted, and better
maintained
- This allows us to focus on the core Llama Stack functionality rather
than reimplementing model management tools

Changes made:
- Removed all model-related CLI commands and their implementations
- Updated documentation to recommend using `huggingface-cli` for model
downloads
- Removed Meta-specific download logic and statements
- Simplified the CLI to focus solely on stack management operations

Users should now use:
- `huggingface-cli download` for downloading models
- `huggingface-cli scan-cache` for listing downloaded models

This is a breaking change as it removes previously available CLI
commands.

Signed-off-by: Sébastien Han <seb@redhat.com>

README.md

@@ -25,7 +25,7 @@ pip install -U llama_stack
 
 MODEL="Llama-4-Scout-17B-16E-Instruct"
 # get meta url from llama.com
-llama model download --source meta --model-id $MODEL --meta-url <META_URL>
+huggingface-cli download meta-llama/$MODEL --local-dir ~/.llama/$MODEL
 
 # start a llama stack server
 INFERENCE_MODEL=meta-llama/$MODEL llama stack build --run --template meta-reference-gpu