agents: configured model variant ignored, session falls back to saved preference

Summary

Agent-configured model variant is silently ignored: build is configured
with variant low, but a fresh opencode2 session starts with medium.

Environment

• opencode version: v0.0.0-beta-18999 (opencode2 --version)
• OS: Linux 7.0.0-30-generic (linux x64)
• Terminal: TERM=xterm-256color, COLORTERM=truecolor
• Shell: /bin/bash
• Install/channel: beta
• Active plugins: none configured in ~/.config/opencode/opencode.jsonc

Reproduction

1. Configure the agent with a valid catalog variant:

   "agents": {
     "build": { "model": "xai/grok-4.6", "variant": "low" },
   }
   

   (low is valid: /api/model lists xai/grok-4.6 variants as
   low, medium, high, xhigh.)
2. Confirm the agent record picked it up:

   $ opencode2 api get /api/agent | python3 -c "
   import json,sys
   for a in json.load(sys.stdin)['data']:
     if a['id']=='build': print(json.dumps(a['model']))"
   {"id": "grok-4.6", "providerID": "xai", "variant": "low"}
   

3. Start opencode2. The footer shows:
   Build · Grok 4.6 xAI · medium

Expected Behavior

Session uses the agent-configured variant low.

Actual Behavior

Session uses medium — the stale value from
~/.local/state/opencode/model.json ("xai/grok-4.6":"medium").
TUI variant resolution (packages/opencode/src/cli/cmd/run/variant.shared.ts
resolveVariant: CLI flag → saved preference → session history) never
consults the agent config, and session prompt creation
(packages/opencode/src/session/prompt.ts createUserMessage) only reads a
top-level agent.variant, while the agent record carries the variant nested
under model. Either syntax (provider/model#variant or "variant" field)
ends up ignored.

Additional Context

• Explicit session variant works fine (POST /api/session/{id}/model with a
  variant sticks), so only the agent → session handoff drops it.
• Reproducible consistently.