fix(provider): pin per-model default params on subagent turns

[justin-carper]

Problem

Subagents that don't set their own model inherit the invoking primary agent's model (per opencode's documented resolution). When that model is a Cursor model like composer-2.5, the subagent turn reaches the provider with the model's opencode options.params dropped, so resolveControls produced a bare { id: "composer-2.5" } with no fast param — and Cursor's server-side fast: true default silently applied. Result: subagents ran "composer-2.5 fast" even though the plugin pins fast off on the normal chat path.

The model choice itself (inheritance) is correct/expected. The bug was only the fast leak on the inherited path.

Fix

• resolveControls floor (controls.ts): StaticControls.defaults applied under static params and per-request options (defaults < params < per-request). On the subagent path (params dropped) the floor re-applies fast: "false"; an explicit fast variant or static param still wins.
• Plumbing (plugin/index.ts → provider/index.ts → language-model.ts): the config hook builds a modelId → defaultModelParams map from the discovered catalog (reusing existing defaultModelParams) and injects it into provider options — a per-provider channel that survives opencode's subagent param-drop. The provider passes it as the defaults floor for the current model.
• Confirmation instrument: OPENCODE_CURSOR_DEBUG=1 now logs the resolved modelSelection per turn, so the leak (and its fix) is observable.

Verification

• npm test → 306 passed (27 files), including:
  • new resolveControls cases: floor applies when no params; per-request and static params override the floor;
  • new end-to-end language-model test: a params-less turn sends Agent.create { id: "composer-2.5", params: [{ id: "fast", value: "false" }] }.
• npm run build (tsup) → clean.
• tsc --noEmit: no errors in changed files. (Pre-existing @types/semver errors in version-check.* are unrelated and reproduce on main.)

Notes

• No behavior change on the normal chat path (opencode already forwards options.params there, so re-applying the floor is a no-op).