Add an OpenCode Go preset, with the x-opencode-session header it requires

[rodrigo103]

What

Adds an OpenCode Go preset (opencode.ai/zen/go) to the provider registry: chat-only, dynamic models, curated defaults, session header included.

Why a preset and not "Add your own server"

Since 2026-09-03 Go requires every request to carry a stable x-opencode-session id per conversation, and has errored without one since 2026-09-06:

request is missing x-opencode-session and cannot be routed efficiently

(reported from the app against a live Go key, 2026-10-03). A user-defined endpoint cannot set headers in this app, but a preset's extraHeaders can — the same mechanism OpenRouter's X-Title already uses, so the id lives there. A keyboard sends single-shot rewrites rather than a conversation, so one fixed, stable id is the honest reading of "stable" and gives routing/caching a consistent key; it is the same shape of key the third-party proxies of this service inject.

Models

Curated ids were read off the live catalog (GET /zen/go/v1/models) and Go's endpoint table, both on 2026-10-03, restricted to the /chat/completions family the client speaks. The catalog also lists /messages (MiniMax, Qwen Max/Flash, Qwen Plus) and /responses (Grok, GPT Lunas, Muse Spark) models that would fail through this client. The live catalog is still merged on top, so newer chat models need no app update.

Verification

• New unit test OpenCodeGoPresetTest: the preset carries the session header, and is chat-only.
• :lib:dictate-core:testDebugUnitTest and :app:testDebugUnitTest --tests "*ProviderListingTest" pass.
• End-to-end verified with a real subscription key: this branch was built, installed on a Galaxy Z Fold4 (Android 16), and rewording through the preset works. (For the record, the endpoint's auth check precedes the session check — four unauthenticated probes with and without the header all answer 401 auth errors — so this had to be proven with a live key.)

Deliberately not attempted: hiding the /messages and /responses ids from the fetched picker list. The catalog carries no endpoint information, and guessing abilities from names is what #313 removed — happy to add a mechanism if you want one.

[rodrigo103]

Live verification done: built this branch, installed it on a Galaxy Z Fold4 (Android 16), configured the preset with a real OpenCode Go key — rewording works first try. Marking ready for review.