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/clients.ts

@@ -0,0 +1,32 @@
+import type { BackgroundManager } from "../../features/background-agent"
+
+export type BackgroundOutputMessage = {
+  id?: string
+  info?: { role?: string; time?: string | { created?: number }; agent?: string }
+  parts?: Array<{
+    type?: string
+    text?: string
+    thinking?: string
+    content?: string | Array<{ type: string; text?: string }>
+    output?: string
+    name?: string
+  }>
+}
+
+export type BackgroundOutputMessagesResult =
+  | { data?: BackgroundOutputMessage[]; error?: unknown }
+  | BackgroundOutputMessage[]
+
+export type BackgroundOutputClient = {
+  session: {
+    messages: (args: { path: { id: string } }) => Promise<BackgroundOutputMessagesResult>
+  }
+}
+
+export type BackgroundCancelClient = {
+  session: {
+    abort: (args: { path: { id: string } }) => Promise<unknown>
+  }
+}
+
+export type BackgroundOutputManager = Pick<BackgroundManager, "getTask">