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/session.ts

@@ -1827,6 +1827,7 @@ export class Session extends EventEmitter {
       ompConfig: this._ompConfig,
       resumeSessionId: this._resumeSessionId,
       effort: this._effort,
+      model: this._model,
       customModel: this.customModel,
       // COD-118: runtime-only — surfaced so the frontend can require explicit user
       // intent before restarting a crash-looped session. Deliberately NOT restored