feat(copilot): add independent context window selection in v2 - #54782

I couldn't check this end to end: the context picker only appears for a GitHub Copilot model with a long-context price tier, and that needs real Copilot credentials. The PR's core tests (30) and TUI tests (22) pass, and typecheck passes in packages/core and packages/tui. I couldn't produce screenshots for the same reason. The picker screenshots in the description cover the new dialog; a before/after of the prompt footer and sidebar cost line would help.

Public API (adds to the API and removes nothing, but clients will see the extra variants):

• Model.Info gains an optional context: { default: number, long: number } in the schema, OpenAPI files and generated client.
• Copilot models that have a long-context tier now list extra variants <effort>@default / <effort>@long (for example default@long and high@default) next to the reasoning variants.
• The stored variant string can now be effort@tier, also through --model provider/model#variant.
• New TUI command context.list with the /context-window slash command.

Concerns

1. The combined variants show up in other clients. The TUI filters them out, but the web/desktop app lists Object.keys(model.variants) as-is (packages/app/src/providers/models/selection.tsx). For a Copilot model with low/medium/high reasoning, the app would offer eleven choices (default@default, low@long, high@default…), and picking one there changes the context window without saying so. Can the context tier be kept out of the variant list, for example by resolving effort@tier in the resolver without adding every combination to model.variants? If not, the app needs the same filter and picker.
2. Default behaviour changes for Copilot users. Users who haven't chosen anything are now held to the smaller default budget (withVariant uses context.default when no variant is selected), so their sessions compact earlier than before. That may be intended, matching Copilot CLI, but it should be called out.

Details are in the inline comments.

Related: #38164, #52089

Both concerns from my last review are resolved:

• Variants listed in other clients: Copilot models now list only their reasoning variants. provider.ts no longer adds tier combinations or skips Variant.resolve. The resolver, the subagent tool, the TUI and the app accept effort@tier through the shared @opencode/util/model-variant helper.
• Default budget change: models.mdx now says Default is used unless Long is selected, and that existing sessions compact earlier.

I couldn't run it end to end, because the picker only appears for a Copilot model that advertises separate input budgets, which needs Copilot credentials. The tests nearest the change pass (core 89, TUI 33, app 10), and typecheck passes in core, tui, app and util.

Public API (backwards compatible): Model.Info gains an optional context: { default, long } in the schema, OpenAPI files and generated client. The variant reference accepted by the server, the CLI #variant syntax and the subagent tool now also accepts <effort>@default|long for models with context. There's a new TUI command, context.list (/context-window). Copilot model variant lists are unchanged.

One small point that doesn't block a merge: the new app test (preserves combined saved selections… in packages/app/src/providers/models/variant.test.ts) packs ten edge-case assertions into one case. It would be easier to follow cut down to the core cases: keeping a saved effort@long, rejecting an unknown effort, and cycling effort without changing the tier.

Related: #38164, #52089