fix(core): preserve shell output tail

Shell truncation read from cursor zero, so large command output kept the head instead of the useful tail. Read the final capture window and cover the direction with head/tail markers.

packages/schema/src/shell.ts

@@ -65,6 +65,7 @@ export interface CreateInput extends Schema.Schema.Type<typeof CreateInput> {}
 export const OutputInput = Schema.Struct({
   cursor: optional(NonNegativeInt),
   limit: optional(NonNegativeInt),
+  keep: optional(Schema.Literals(["head", "tail"])),
 })
 export interface OutputInput extends Schema.Schema.Type<typeof OutputInput> {}