fix(app): restore saved model & permission picks in new session

[jlixfeld]

Problem

The new-session screen persists the user's model and permission picks to the draft (MMKV) — useNewSessionDraft even documents that it restores them — but it never reads them back. The mount effect always reseeds the picker indices from the agent defaults, so a selection silently resets to the default on every remount. This is most visible on iOS, where navigation remounts the screen, so users report "iOS doesn't save model settings." Machine / path / agent restore correctly; model and permission don't.

Root cause

new/index.tsx reset effect:

const defaultModelIdx = modelModes.findIndex(m => m.key === effectiveAgentDefaults.modelMode);
setModelIndex(defaultModelIdx >= 0 ? defaultModelIdx : 0);

draft.modelMode / draft.permissionMode are written on pick but never consulted here. The draft's initial 'default' value also collides with the literal 'default' model/permission key, so "never picked" couldn't be told apart from an explicit "default" pick.

Fix

• Make the draft's modelMode / permissionMode nullable (null = unset), so an explicit "default" pick is distinct from never having picked.
• Add a pure, tested resolveModeIndex(modes, draftKey, fallbackKey) helper: prefer the saved pick when still available for the current agent, else the agent default, else index 0.
• Use it in the reset effect (model + permission), with the draft values added to the dependency list.

Selecting a different agent still falls back to that agent's default (a pick from another agent won't be in its mode list). Setting a global default in Settings → Agent Defaults still applies for users who never picked in the new-session screen.

Tests

resolveModeIndex.test.ts covers: restore saved pick, explicit-default vs unset, fallback to agent default, cross-agent fallback, and the index-0 floor. Full suite green; pnpm typecheck clean.

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