feat(preemptive-compaction): implement automatic session compaction at token threshold

Monitor token usage after assistant responses and automatically trigger session
compaction when exceeding configured threshold (default 80%). Toast notifications
provide user feedback on compaction status.

Controlled via experimental.preemptive_compaction config option.

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

src/config/schema.ts

@@ -110,6 +110,10 @@ 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(),
+  /** Threshold percentage to trigger preemptive compaction (default: 0.80) */
+  preemptive_compaction_threshold: z.number().min(0.5).max(0.95).optional(),
 })
 
 export const OhMyOpenCodeConfigSchema = z.object({