refactor: wave 2 - split atlas, auto-update-checker, session-recovery, todo-enforcer, background-task hooks

- Extract atlas/ into 15 focused modules (hook, event handler, tool policies, types, etc.)
- Split auto-update-checker into checker/ and hook/ subdirectories with single-purpose files
- Decompose session-recovery into separate recovery strategy files per error type
- Extract todo-continuation-enforcer from monolith to directory with dedicated modules
- Split background-task/tools.ts into individual tool creator files
- Extract command-executor, tmux-utils into focused sub-modules
- Split config/schema.ts into domain-specific schema files
- Decompose cli/config-manager.ts into focused modules
- Rollback skill-mcp-manager, model-availability, index.ts splits that broke tests
- Fix all import path depths for moved files (../../ -> ../../../)
- Add explicit type annotations to resolve TS7006 implicit any errors

Typecheck: 0 errors
Tests: 2359 pass, 5 fail (all pre-existing)

src/tools/background-task/session-messages.ts

@@ -0,0 +1,22 @@
+import type { BackgroundOutputMessage, BackgroundOutputMessagesResult } from "./clients"
+
+export function getErrorMessage(value: BackgroundOutputMessagesResult): string | null {
+  if (Array.isArray(value)) return null
+  if (value.error === undefined || value.error === null) return null
+  if (typeof value.error === "string" && value.error.length > 0) return value.error
+  return String(value.error)
+}
+
+function isSessionMessage(value: unknown): value is BackgroundOutputMessage {
+  return typeof value === "object" && value !== null
+}
+
+export function extractMessages(value: BackgroundOutputMessagesResult): BackgroundOutputMessage[] {
+  if (Array.isArray(value)) {
+    return value.filter(isSessionMessage)
+  }
+  if (Array.isArray(value.data)) {
+    return value.data.filter(isSessionMessage)
+  }
+  return []
+}