agents.<name>.model is ignored when resolving the primary session model

[shuv1337]

Summary

A configured agents.build.model (e.g. openai/gpt-5.6-sol) is not used for the primary session's model. With no top-level model and no explicit model on session create, the runner falls back to the catalog default — "newest released available model" — which on a fresh install is a free opencode/* model.

Observed (integration-v2 @ 4668dfc, shuvcode serve --service)

• GET /api/agent reports {"id":"build","model":{"id":"gpt-5.6-sol","providerID":"openai"}}.
• A session created without model and prompted normally ran on {"id":"nemotron-3.5-lightning-free","providerID":"opencode"} (which then looped on provider.invalid-output).

Code path

• packages/core/src/session/context.ts:120 → models.resolve(selection.session)
• packages/core/src/session/runner/model.ts:80-83 — if (!session.model) resolver.resolve(); selection.agent.info.model is never consulted.
• packages/core/src/model-resolver.ts:356-366 → catalog.model.default()
• packages/core/src/catalog.ts:196-205 — no configured default ⇒ model.available()[0], sorted by time.released desc (catalog.ts:180-184).
• Contrast: the command path does honor the agent model — packages/core/src/session.ts:710: command.model ?? commandAgent?.model ?? input.model.
• Session.create only stores input.model (session.ts:417-421); switchAgent doesn't touch the model (session.ts:805-811).

Expected

Resolution order session.model ?? agent.model ?? config.model ?? catalog default, consistent with the command path.

Workaround

Set top-level "model" in opencode.json, or pass model on POST /api/session.

Found while running the Discord plugin (shuvcode-bot) on a fresh VM.