feat: add free model resolution

[caretak3r]

Issue for this PR

Closes #21863

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds Provider.resolveSelection() which resolves --model free to a random free opencode-provider model. With --variant any, a random variant is chosen from that model's available variants (and falls through cleanly when the model has none).

Selection logic (packages/opencode/src/provider/provider.ts:1742-1780):

// Eligible iff providerID === opencode AND every cost field is zero
//   AND (id === "big-pickle" || id.endsWith("-free"))
isFree(model) && isListed(model)

The providerID === opencode gate excludes OpenRouter :free models. The isListed gate excludes any opencode-provider model that happens to have zero declared cost but isn't actually meant to be in the free pool (e.g. gpt-5-nano).

Integration points:

• cli/cmd/run.ts — resolves before prompting / sending command
• cli/cmd/tui/thread.ts — resolves before launching the TUI
• cli/cmd/tui/app.tsx + context/args.tsx — store the resolved variant in local state

How did you verify your code works?

Workspace gates (run locally on this branch, all green):

Check	Result
bun turbo typecheck	13/13 packages pass
bun test (the 3 affected files)	88 / 88 pass, 238 expectations
Pre-push husky hook (typecheck)	passes

Named tests covering this feature —

In packages/opencode/test/provider/provider.test.ts:

• resolveModel picks only valid opencode free listings — proves the filter admits only (providerID = opencode) ∩ (cost = 0) ∩ (id = big-pickle | endsWith("-free")), and explicitly rejects gpt-5-nano and OpenRouter free-router.
• resolveSelection picks a variant from the chosen free model — Math.random is mocked to land on a model with variants; the picked variant comes from that model's own variant set.
• resolveSelection keeps explicit variants unchanged — --variant max (literal) is preserved verbatim, not re-randomized.
• resolveSelection falls back to no variant when the chosen free model has none — graceful degrade for models that have no variants.

In packages/opencode/test/cli/cmd/run.test.ts:

• resolves free before prompting
• passes the resolved model to command sessions
• passes the resolved any variant to sessions

In packages/opencode/test/cli/tui/thread.test.ts:

• resolves the free model alias before launching the tui
• passes the resolved any variant before launching the tui

Negative cases verified — --model free excludes:

• OpenRouter :free / -free aliases (different providerID)
• opencode/gpt-5-nano (zero cost but not in isListed whitelist)
• Any model with non-zero cost.input, cost.output, cost.cache.read, cost.cache.write, or experimentalOver200K

Manual sanity — Explicit model passthrough (--model opencode/big-pickle) is unchanged because resolveSelection short-circuits when the model is not literally "free". --variant any without --model free is also short-circuited.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[caretak3r]

@rekram1-node @adamdotdevin Anything for me to do here, looks like other changes were pushed here? Thank you!

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