[codex] Add /effort slash command

[Jaywalk18]

Summary

• add /effort [level] as a text shortcut for changing the current Codex session reasoning effort
• validate requested effort levels against the current model's supported efforts from the Codex app-server model list
• document /effort high in help/welcome cards and README

Notes

• /model keeps the existing card-based model + effort picker.
• /effort only updates persisted sessions; if a session is not initialized yet it now explains that instead of pretending the change was saved.
• Changes take effect on the next turn, matching the existing per-turn runStreamed(..., { model, effort }) behavior.

Tests

• npm run typecheck
• npm test
• npm run build