refactor(agent-model): move Session model selection into its own package

Add @deepseek-ai/dsh-agent-model. It provides ctx.agentModel
(resolveForNextStep, select, applyConfiguredRoute) and owns the
modelSelection projection, which the Agent registry no longer registers.
dsh-agent-loop now injects agentModel; the base and SDK-minimal bundles
and the Python SDK runtime include it.

The provider keeps the projection registered until consumers that are
still unloading finish their cleanup. The model-switch notice becomes an
opt-in per-Agent plugin at @deepseek-ai/dsh-agent-model/notice, and
agent/request now also rejects reasoning-effort changes after step
preparation.

Mount the provider in every composition that loads dsh-agent-loop, and
update consumers, the switch-notice snapshot driver, docs, and the
upgrade guide.

python/sdk-runtime/package.json

@@ -17,6 +17,7 @@
     "@deepseek-ai/dsh-agent-default-model": "workspace:*",
     "@deepseek-ai/dsh-agent-instructions": "workspace:*",
     "@deepseek-ai/dsh-agent-loop": "workspace:*",
+    "@deepseek-ai/dsh-agent-model": "workspace:*",
     "@deepseek-ai/dsh-agent-preset-registry": "workspace:*",
     "@deepseek-ai/dsh-agent-tool-presentation": "workspace:*",
     "@deepseek-ai/dsh-anonymous-user-id": "workspace:*",