Do not include the active model in the system prompt

[luw2007]

The active model is currently included in the system prompt as part of the workstation context:

<workstation>
OS: ...
Arch: ...
Model: ...
</workstation>

This is unnecessary and causes avoidable prompt-cache invalidation.

The selected model is a request-routing setting, not project context or workstation context. Switching models is a normal operation, but it currently changes the system prompt. Since the model line appears before other stable prompt content, a model switch can prevent reuse of cached prompt prefixes for repository rules, workspace context, and similar stable content.

The simplest fix is to remove Model: ... from the system prompt.

No replacement runtime block is needed unless there is a concrete product requirement for the agent to know its own model name.

After this change:

• model switching only changes the outbound request configuration;
• the stable system prompt remains unchanged;
• no prompt/model synchronization is needed after model switches or compaction;
• compaction cannot leave a stale or incorrect model name in the prompt.

Suggested regression tests:

1. The system prompt does not contain Model:.
2. Switching models changes the model used for the next request.
3. Compaction does not change the active model used for subsequent normal requests.