decision: session mode/model controls vs the documented "no model switcher" constraint

Extracted from #39, which adds mode/model selectors to the composer. The project has so far deliberately kept agent config out of the UI ("agent lives in goose"), while Goose Desktop's composer has exactly these controls; desktop parity is the argument in favor.

Decide whether the constraint stands or is revised. No code in this issue; the implementation is tracked separately and gated on this decision.

**Tier**: 1 (decision)