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/web/schemas.ts

@@ -529,12 +529,15 @@ export const CreateSessionSchema = z.object({
   /**
    * Claude model for THIS session only, passed as `claude --model <id>`; nothing is written to
    * disk. Wins over the app-wide default model. Same character set as the registry's
-   * `model-claude` pattern, so a value accepted here is never rejected at launch.
+   * `model-claude` pattern, so a value accepted here is never rejected at launch. An empty
+   * string means no per-session model, as it does for `modelOverride`. Claude only: the route
+   * refuses it for any other CLI.
    */
   model: z
     .string()
     .max(100)
     .regex(/^[a-zA-Z0-9._\-[\]]+$/)
+    .or(z.literal(''))
     .optional(),
   openCodeConfig: OpenCodeConfigSchema,
   codexConfig: CodexConfigSchema,