/model: add session-only flag to avoid overwriting saved default

Problem

Running /model <name> inside a session both switches the active model and saves it as the default for new sessions. There's no way to temporarily switch models mid-session without overwriting settings.json.

Expected behavior

A way to switch the model for the current session only, leaving the saved default untouched. For example:

• A flag: /model --session claude-opus-4-8
• Or a separate command: /model:session <name>

Current workaround

None known. Even setting ANTHROPIC_MODEL env var, /model still writes the new default.

Use case

Switching to a more capable model for one complex task, then having the next session start with the usual (cheaper/faster) default — without manually resetting it each time.