Model switches return error codes and skip the server check for a model picked at launch

What

• A failed model switch now returns a machine-readable code. The code is one of alias_1m_disabled, alias_1m_unsupported, restricted_by_org, unavailable_for_account, not_offered, check_failed or, when a hook blocked the switch, blocked_by_hook. A hook is a command you configure to run on events.
• Error messages mention /model only where that command exists. Elsewhere, they say "Switch to a different model." or just "Try again." Before, they always said to run /model.
• The "Couldn't confirm model" error is now marked retryable.
• A model picked at launch, for example by the SDK or in print mode (-p), is no longer checked against the server. The switch passes confirmWithServer: false, which becomes serverProbe: false, and the model is accepted as valid. The server check is also skipped when no deadline is given.
• The print and SDK session now reports when a pick is a launch pick (isLaunchModelPick).

Why

The SDK is the library for running Claude Code from your own programs, and programs that embed Claude Code often have no /model command. Those hosts now get messages that make sense and a code they can act on. Starting with a model chosen at launch no longer waits on a server check, which matters most on a slow network.