fix: add compacting emoji to config schema and zod validation

Allow users to customize the compacting status reaction emoji via
messages.statusReactions.emojis.compacting in config.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/config/types.messages.ts

@@ -58,6 +58,7 @@ export type StatusReactionsEmojiConfig = {
   error?: string;
   stallSoft?: string;
   stallHard?: string;
+  compacting?: string;
 };
 
 export type StatusReactionsTimingConfig = {

src/config/zod-schema.session.ts

@@ -169,6 +169,7 @@ export const MessagesSchema = z
             error: z.string().optional(),
             stallSoft: z.string().optional(),
             stallHard: z.string().optional(),
+            compacting: z.string().optional(),
           })
           .strict()
           .optional(),