fix(models): default Cursor fast tier OFF, add picker toggle

[justin-carper]

Problem

The variant builder (buildModelVariants) only mapped reasoning/effort params (/think|reason|effort/) and dropped Cursor's fast toggle entirely, so it never reached providerOptions.cursor.

Cursor's catalog marks the default variant of several models as fast: true — composer-2.5, composer-2, and the gpt-*-codex line. Because the provider omitted the param, opencode silently ran the fast tier with no way to opt out from the picker.

Fix

fast now defaults off:

• New defaultModelParams() pins non-reasoning boolean params (fast) OFF.
• Fast-capable models seed options.params.fast = "false" (via toOpencodeModels + buildModelV2Map), sent every turn through the existing options.params → providerOptions.cursor.params channel resolveControls already reads.
• Each reasoning variant bakes in fast: "false", so picking a reasoning level can't silently re-enable fast (robust regardless of how opencode merges variant vs. options params).
• A fast picker variant lets you opt back in.
• Override per model: provider.cursor.models.<id>.options.params.fast.

No resolveControls change required.

Deliberate constraint

opencode variants are single-select, so you can't pick fast and a reasoning level from the picker at once — use static options.params config for that combo (noted in README).

Tests

TDD (tests written and watched fail first):

• test/model-variants.test.ts — fast opt-in toggle, default-off, baked-in reasoning variants, defaultModelParams.
• test/model-discovery.test.ts — toOpencodeModels seeds options.params.
• test/model-v2.test.ts (new) — buildModelV2Map seeds options.

tsc --noEmit clean · 192/192 tests pass · tsup build success.

Docs updated (README per-request controls + CHANGELOG Unreleased).