[Feature]: Separate OpenCode reasoning effort and agent (Build/Plan) into distinct composer controls #14210

Area

apps/web, apps/server

Problem / Observed Behavior

When using the OpenCode provider in T3 Code, the reasoning effort (model variant, e.g. Low / Medium / High / Max) and the agent mode (Build / Plan) are bundled into a single compound dropdown trigger in the composer footer (e.g. Max · Build or Medium · Build).

Clicking this trigger opens a single shared popover menu grouping both Reasoning and Agent together:

+-----------------------+
| Reasoning             |
|   Low                 |
|   High                |
|   ✓ Max               |
+-----------------------+
| Agent                 |
|   ✓ Build [Default]   |
|   Plan                |
+-----------------------+

This contrasts with Codex and Claude, where reasoning/effort is exposed as its own dedicated input control separate from interaction/mode controls.

Background

In PR #9287 (fix(opencode): show Reasoning selector for OpenCode models, addressing #2144), the OpenCode variant descriptor was renamed to Reasoning and synthesized into the provider's optionDescriptors. Because TraitsPicker joins all select option descriptors together (labels.join(' · ')), OpenCode's reasoning and agent options ended up rendered as a single joint trigger (Max · Build).

Expected Behavior

Reasoning effort and Agent selection should be distinct, separate controls in the composer:

1. Dedicated Reasoning Effort picker: A standalone dropdown for reasoning levels (Low, Medium, High, Max / xhigh), matching the UX pattern of Codex and Claude.
2. Dedicated Agent / Mode control: A separate control for OpenCode agent selection (Build, Plan), aligned with how interaction modes are handled.

Steps to Reproduce

1. Open T3 Code composer.
2. Select the OpenCode provider and any model supporting variants (e.g., DeepSeek V4.1 Flash).
3. Observe the footer toolbar: instead of a dedicated reasoning selector, a single joint pill (Max · Build) is displayed that bundles both Reasoning and Agent options into one menu.

Why this matters

• Cross-provider consistency: Users switching between Codex, Claude, and OpenCode expect reasoning controls to be in a familiar, dedicated location.
• Discoverability: Bundling reasoning into the agent/mode dropdown makes reasoning effort hard to discover, leading users to believe effort adjustment is unsupported or fixed to the default.
• UX Clarity: Reasoning depth and agent mode are orthogonal settings that should not require opening a compound popover to change.