Allow excluding specific models from Auto model selection #5231

Describe the feature or problem you'd like to solve

exclude models from AUTO that fail regularly, only use these explicitly

Proposed solution

Problem
When Auto is on, Copilot CLI may route a request to a model that is unusable for my workflow. For example, a model that is slow, times out, or reliably fails on large agentic tasks.
I can't stop Auto from choosing it. The only choice is to give up Auto and pin one model with  /model  or  /config model .

Request
A user-level setting, with an optional repo-level override, that lists models Auto must never pick. For example  autoExcludeModels  in the user config, available through  /settings .
A manual  /model   would still work for an excluded model, so I can use it deliberately when it suits the task.

Expected behaviour

• Auto picks only from the available models that are not excluded.
• If every candidate is excluded, the CLI shows a clear message instead of silently picking an excluded model.
•  /settings  shows the exclusion list, and  /settings unset  clears it.
• The setting also applies to models Auto chooses for subagents.

Why not alternatives

• Pinning one model gives up Auto's per-task routing.
• Memories or custom instructions can't help, because routing happens before the model sees any instructions.

Acceptance criteria

1. A model listed in the setting is never auto-selected, in the main session or in subagents.
2. The same model can still be chosen explicitly.
3. The setting is documented in  /help  and in the CLI docs.

Example prompts or workflows

No response

Additional context

No response