refactor: split CLI, Desktop, and account store module boundaries

[Dikevin]

Summary

This PR is a structural refactor only. It does not intentionally change CLI or runtime semantics.

No Semantic Changes

• no new user-facing features
• no intended behavior changes in account switching, Desktop launch/watch flows, or runtime reads
• changes are focused on extracting modules, clarifying boundaries, and organizing code into shallow subsystem directories

What Was Refactored

• extracted CLI desktop/watch orchestration out of src/main.ts into focused modules such as src/commands/desktop.ts, src/switching.ts, src/watch/session.ts, src/watch/output.ts, and src/desktop/managed-state.ts
• split src/codex-desktop-launch.ts internals into helper modules for devtools access, runtime expression/normalization, process helpers, shared constants/utilities, managed state parsing, and shared types
• split account-store helpers into dedicated modules for repository access, config snapshot handling, storage utilities, and shared account-store types
• grouped the account-store, desktop, and watch implementations into shallow subsystem directories under src/ and updated the codebase to import the directory-backed modules directly
• tightened and stabilized runner/watcher tests to make the refactor easier to verify

Verification

• pnpm typecheck
• pnpm test