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/shared/command-executor/shell-path.ts

@@ -0,0 +1,27 @@
+import { existsSync } from "node:fs"
+
+const DEFAULT_ZSH_PATHS = ["/bin/zsh", "/usr/bin/zsh", "/usr/local/bin/zsh"]
+const DEFAULT_BASH_PATHS = ["/bin/bash", "/usr/bin/bash", "/usr/local/bin/bash"]
+
+function findShellPath(
+	defaultPaths: string[],
+	customPath?: string,
+): string | null {
+	if (customPath && existsSync(customPath)) {
+		return customPath
+	}
+	for (const path of defaultPaths) {
+		if (existsSync(path)) {
+			return path
+		}
+	}
+	return null
+}
+
+export function findZshPath(customZshPath?: string): string | null {
+	return findShellPath(DEFAULT_ZSH_PATHS, customZshPath)
+}
+
+export function findBashPath(): string | null {
+	return findShellPath(DEFAULT_BASH_PATHS)
+}