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>

.github/workflows/python-build-test.yml

@@ -43,7 +43,5 @@ jobs:
         uv pip list
         uv pip show llama-stack
         command -v llama
-        llama model prompt-format -m Llama3.2-90B-Vision-Instruct
-        llama model list
         llama stack list-apis
         llama stack list-providers inference