feat: add variant support for subagents (#7138) [alt of #7140]

[CasualDeveloper]

What does this PR do?

Adds agent-level default variant configuration, allowing users to set reasoning effort (e.g., "low", "medium", "high") per agent. This is an alternative to #7140 with broader scope: full desktop/web support, dynamic Task tool overrides, and TUI CLI parity.

Why this matters: Orchestrator agents can now spawn subagents with different model/variant combinations dynamically — no need to duplicate agent configs for each reasoning level.

Changes

Core backend

• variant field in agent config schema
• Resolution order: user input > agent config
• Resolved variant stored in message metadata (not undefined when using agent default)
• Subagents use their own configured variant, not inherited from parent

Task tool dynamic override (addresses @terwey's feedback on #7140)

• Optional model and variant params added to Task tool schema
• Priority: param override > agent config > parent context (model) / undefined (variant)

TUI

• Effective variant displayed in prompt bar and agent picker
• Added --variant CLI flag for parity with headless run command

Desktop/web app

• Effective variant displayed in prompt bar and agent selector

SDK

• Updated types.gen.ts and openapi.json

Comparison with #7140

Feature	#7140	This PR
Core backend	Yes	Yes
TUI display	Yes	Yes
Desktop/web app	No	Yes
Message metadata	undefined when agent default used	Resolved variant stored
Task tool model/variant override	No	Yes
TUI --variant CLI flag	No	Yes
Files changed	9	18

Example usage

Agent config:

# .opencode/agent/oracle.md
---
model: anthropic/claude-sonnet-4
variant: "high"
---

Task tool override:

{
  "description": "Refactor auth module",
  "prompt": "Refactor the auth module to use OAuth2",
  "subagent_type": "implementer",
  "model": "anthropic/claude-sonnet-4-20250514",
  "variant": "low"
}

TUI CLI:

opencode --variant high

Closes #7138