feat(vscode): add opencode model effort toggle

[zeug-zz]

Adds optional explicit model effort/variant selection for reasoning-capable models in the OpenCode chat webview, matching the opencode TUI Ctrl+T flow. When the selected model advertises variants (e.g. low / medium / high), the user can cycle through them with Ctrl+T in the textarea; the chosen effort is forwarded on sendMessage and editAndResend payloads only when explicitly set — omitting the field entirely when unset so the server applies its own default.

What changes

• Core (@opencodegui/core): add optional ModelVariantRef to ModelInfo.variants, SendMessageOptions.effort, and UIToHostMessage sendMessage / editAndResend. New helpers getModelVariants, isModelVariantSupported, validateModelVariant with 27 unit tests covering unsupported metadata, disabled variants, and model-change invalidation. Add vitest config for the core package.
• Agent (opencode): sendMessage forwards options.effort.id as a top-level variant on client.session.promptAsync (sibling of model). Default payloads omit the key entirely. executeShell is intentionally unchanged — SDK 1.2.17 shell body has no variant field.
• Webview: useProviders tracks selectedModelEffort, validates on model change, and clears when the new model does not advertise the prior id. ModelSelector displays effort (label or id) compactly next to the model name with a middle-dot separator; hidden when unset. InputArea handles Ctrl+T (textarea only, no meta/alt modifiers) to cycle valid variants; preventDefault fires only when a cycle action is performed.
• App: forwards effort in sendMessage / editAndResend payloads only when selected; executeShell payload remains { sessionId, command, model }.
• Host: chat-view-provider forwards message.effort into IAgent.sendMessage options for send and edit-resend; never for executeShell.
• Locales updated: en, es, ja, ko, pt-br, ru, zh-cn, zh-tw.

Impact

• 28 files changed, +2697 / −34
• Tests: 1720/1720 pass (27 new core tests + new scenario 26-effort-cycle.test.tsx + expanded coverage in messaging, editing, shell, model-selection, useProviders, and ModelSelector tests)
• pnpm check introduces no new warnings in the touched files
• No new runtime dependencies
• No breaking changes to existing model selection, message sending, shell execution, or Markdown rendering
• executeShell intentionally omits effort — SDK 1.2.17 has no variant field on the shell body