feat: add agent fallback and preemptive-compaction restoration

- Add agent visibility fallback for first-run scenarios
- Restore preemptive-compaction hook
- Update migration and schema for preemptive-compaction restoration

src/config/schema.ts

@@ -68,6 +68,7 @@ export const HookNameSchema = z.enum([
   "empty-task-response-detector",
   "think-mode",
   "anthropic-context-window-limit-recovery",
+  "preemptive-compaction",
   "rules-injector",
   "background-notification",
   "auto-update-checker",
@@ -247,6 +248,7 @@ export const DynamicContextPruningConfigSchema = z.object({
 export const ExperimentalConfigSchema = z.object({
   aggressive_truncation: z.boolean().optional(),
   auto_resume: z.boolean().optional(),
+  preemptive_compaction: z.boolean().optional(),
   /** Truncate all tool outputs, not just whitelisted tools (default: false). Tool output truncator is enabled by default - disable via disabled_hooks. */
   truncate_all_tool_outputs: z.boolean().optional(),
   /** Dynamic context pruning configuration */