fix(migration): add hook rename and removal mappings for v3.0.0 upgrade

- Add sisyphus-orchestrator → atlas hook rename mapping
- Add null mappings for removed hooks (preemptive-compaction, empty-message-sanitizer)
- Update migrateHookNames() to filter out removed hooks and return removed list
- Log warning when obsolete hooks are removed from disabled_hooks
- Add tests for new migration scenarios

src/shared/migration.ts

@@ -36,9 +36,15 @@ export const BUILTIN_AGENT_NAMES = new Set([
 ])
 
 // Migration map: old hook names → new hook names (for backward compatibility)
-export const HOOK_NAME_MAP: Record<string, string> = {
+// null means the hook was removed and should be filtered out from disabled_hooks
+export const HOOK_NAME_MAP: Record<string, string | null> = {
   // Legacy names (backward compatibility)
   "anthropic-auto-compact": "anthropic-context-window-limit-recovery",
+  "sisyphus-orchestrator": "atlas",
+
+  // Removed hooks (v3.0.0) - will be filtered out and user warned
+  "preemptive-compaction": null,
+  "empty-message-sanitizer": null,
 }
 
 /**
@@ -77,19 +83,28 @@ export function migrateAgentNames(agents: Record<string, unknown>): { migrated:
   return { migrated, changed }
 }
 
-export function migrateHookNames(hooks: string[]): { migrated: string[]; changed: boolean } {
+export function migrateHookNames(hooks: string[]): { migrated: string[]; changed: boolean; removed: string[] } {
   const migrated: string[] = []
+  const removed: string[] = []
   let changed = false
 
   for (const hook of hooks) {
-    const newHook = HOOK_NAME_MAP[hook] ?? hook
+    const mapping = HOOK_NAME_MAP[hook]
+
+    if (mapping === null) {
+      removed.push(hook)
+      changed = true
+      continue
+    }
+
+    const newHook = mapping ?? hook
     if (newHook !== hook) {
       changed = true
     }
     migrated.push(newHook)
   }
 
-  return { migrated, changed }
+  return { migrated, changed, removed }
 }
 
 export function migrateAgentConfigToCategory(config: Record<string, unknown>): {
@@ -167,11 +182,14 @@ export function migrateConfigFile(configPath: string, rawConfig: Record<string,
   }
 
   if (rawConfig.disabled_hooks && Array.isArray(rawConfig.disabled_hooks)) {
-    const { migrated, changed } = migrateHookNames(rawConfig.disabled_hooks as string[])
+    const { migrated, changed, removed } = migrateHookNames(rawConfig.disabled_hooks as string[])
     if (changed) {
       rawConfig.disabled_hooks = migrated
       needsWrite = true
     }
+    if (removed.length > 0) {
+      log(`Removed obsolete hooks from disabled_hooks: ${removed.join(", ")} (these hooks no longer exist in v3.0.0)`)
+    }
   }
 
   if (needsWrite) {