Add reasoning effort selection for OpenCode models

[suasgn]

Closes #211.

• Discover OpenCode per-model variants via opencode models --verbose (plain-listing fallback) and expose them as reasoning-effort options, ordered on a canonical effort ladder with unknown ids sorted stably after it.
• Send the selection as the top-level variant on every prompt (default selects the base model); model and variant now apply to the live session without restarting the driver (only access-mode changes restart it).
• Add an explicit Default row to the model picker for OpenCode (localized in en/ja/zh-CN) that clears the effort back to the base model; other providers keep the previous auto-select behavior.