[codex] Add cli-v2 model and session pickers

[roackb2]

Summary

• add cli-v2 model, reasoning effort, and session picker panels
• fetch model options once on startup and filter cached model/session data locally
• expose reasoning picker options from the control-plane runtime context while sourcing per-model support from core ModelPolicyService
• route picker selections through slashCommandExecute so core slash command semantics stay server-owned
• move picker state/keyboard/selection wiring into a cli-v2 hook to keep App.tsx focused on rendering
• remove the OAuth account short id from the cli-v2 status bar
• allow ultrahigh reasoning effort for GPT-5.5/GPT-5.5 Pro through centralized model policy
• document that cli-v2 is actively under development, unreleased, and moving the TUI to an API-only control-plane path
• add a model-policy README defining src/core/llm/models as the source of truth for model-specific settings and policy

Validation

• yarn typecheck
• ./node_modules/.bin/vitest run src/tests/unit/core/llm-factory.test.ts src/tests/unit/core/openai-oauth.test.ts src/tests/unit/core/slash-command-modules.test.ts src/tests/unit/tui/local-commands.test.ts src/tests/unit/cli-v2/control-plane-session-store.test.ts src/tests/unit/cli-v2/picker-service.test.ts
• rg boundary check for cli-v2 core/server/old-cli imports
• rg check for removed ultrahigh reserved hardcoding