feat: set the global default model for the pi agent

[imrj05]

What problem does this solve?

Model and thinking effort are selectable per session/composer, but there is no way to set pi's global default model from Orbit. Changing the default means editing pi's config by hand, and the choice is not reflected anywhere in the app's settings.

Proposed solution

A global default-model setting:

• A control (Models page and/or Agent/General settings) to pick the default model from pi's live catalog (get_available_models).
• Persist it to pi's own global config so terminal and Orbit sessions agree — one source of truth, not an Orbit-only override.
• Optionally set a default thinking effort to pair with it.
• Show the current global default clearly and let new sessions inherit it, while keeping per-session overrides.

Alternatives considered

• Per-session selection only (current behavior) — every new session starts from whatever the last pick was, not a deliberate default.
• Editing pi config externally — not discoverable and error-prone.

Area

Composer / models / thinking