fix(sessions): keep a session's model through recovery, refuse it off claude

SessionState now carries the model a session launched with, and both
recovery constructors (mux recovery and reboot restore) pass it back, so a
recovered session relaunches on the same --model rather than the account
default. A top-level `model` sent with any other CLI is refused, since
those take their model in their own config object, and an empty string
means no per-session model, as it does for modelOverride.

CLAUDE.md now describes both routes for a Claude model. The tests pin
which of `model` and `modelOverride` reaches the launch and which the
case file, and that a model opening with a dash renders as --model's value.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/types/session.ts

@@ -795,6 +795,13 @@ export interface SessionState {
   resumeSessionId?: string;
   /** Claude CLI effort level (soft default via --settings, switchable in-session via /effort) */
   effort?: EffortLevel;
+  /**
+   * The model the session was LAUNCHED with (`--model`): the caller's per-session `model`, or
+   * the app-wide default when there was none. Persisted so a recovered session relaunches on
+   * the same model rather than whatever the default is by then. Not `cliModel`, which is what
+   * the CLI's banner reports.
+   */
+  model?: string;
   /**
    * Custom Model Endpoint Profiles (docs/custom-model-endpoints-plan.md): the custom
    * OpenAI-compatible endpoint (local or cloud) this session's CLI is currently pointed