chore: remove deprecated empty_message_recovery experimental option

The empty message recovery is now enabled by default (no longer experimental).
Removes the config option from schema and all README files.

🤖 Generated with assistance of [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/config/schema.ts

@@ -108,7 +108,6 @@ export const SisyphusAgentConfigSchema = z.object({
 
 export const ExperimentalConfigSchema = z.object({
   aggressive_truncation: z.boolean().optional(),
-  empty_message_recovery: z.boolean().optional(),
   auto_resume: z.boolean().optional(),
   /** Enable preemptive compaction at threshold (default: true) */
   preemptive_compaction: z.boolean().optional(),