v0.12.12 — Custom models and reasoning effort

Add Custom as the final option in the model and reasoning-effort selectors, with text entry available when detection fails or omits a value.

Manual model and effort values can also be saved through configuration commands or YAML. Codex, Claude Code, and Pi receive manual effort values directly and validate whether they support them. Settings still commit atomically for subsequent turns, while service-mode validation and ACP's existing effort limitation remain in place.