feat: add OpenCode Go as a third model router

From the PR description

Summary

Adds OpenCode Go - opencode.ai's low-cost subscription gateway for open coding models (GLM-5, Kimi K3, Qwen3.8 Max, MiniMax-M3, DeepSeek V4, ...) - as a third model router, alongside OpenRouter and the Vercel AI Gateway.

Rebased. This PR originally stacked on #339, which introduced a separate model-registry / OpenAI-compatible-provider foundation. main has since landed its own router layer (llm/openrouter.ts serving both OpenRouter and Vercel, lib/routerModels.ts, user_router_models, RouterSettingsSection). Rather than land a second, competing provider architecture, the branch has been rewritten onto current main in that idiom. It no longer depends on #339 and is a single self-contained commit. It is also rebased onto the #357 design-system refactor, so the new group is declared once in the shared ModelToggleUI primitive that both the web app and the Word add-in now render from.

What's added

Backend

• lib/llm/openrouter.ts - opencode-go joins the RouterProvider union: base URL https://opencode.ai/zen/go/v1 (overridable via OPENCODE_GO_BASE_URL, like the other two), its own key lookup and label. OpenRouter's HTTP-Referer/X-Title attribution headers and its reasoning: {...} payload stay OpenRouter-only; OpenCode Go uses reasoning_effort like Vercel.
• lib/llm/models.ts - providerForModel / resolveModel accept opencode-go/<model>; openCodeGoModelId strips the prefix.
• lib/userApiKeys.ts + migration 20260820_01 - new opencode-go API-key provider (OPENCODE_API_KEY env or encrypted per-user storage); schema.sql updated to match so the schema-drift check stays green.
• routes/models.ts - GET /models/opencode-go returns the user's live catalog (key never leaves the server), deduplicated and sorted by label.
• routes/user.ts - the profile PATCH accepts openCodeGoModels, and the saved selection gates requests through the existing user_router_models choke point, so a hand-crafted request still can't spend the operator's env key on an arbitrary gateway model.

Frontend + Word add-in

• lib/mikeApi.ts - getOpenCodeGoModels(), opencode-go API-key provider, openCodeGoModels on the profile.
• Settings → API Keys gains an OpenCode Go field; Settings → Routers gains an OpenCode Go model picker.
• ModelToggle (web) and the add-in's mirrored catalog gain an "OpenCode Go" group, gated on the user's key.

Design note

A router's slug, model-id prefix and API-key provider name are one string. That invariant already held for openrouter/vercel, so this PR makes the code depend on it explicitly: routerForModelId, the selection lookups, the profile payload fields and the title-model fallback are now driven by ROUTER_SLUGS rather than hard-coded pairs. A fourth router should be mostly a list entry plus a label.

The one place the routers genuinely differ is catalog-id shape: OpenRouter and Vercel publish vendor/model pairs, OpenCode Go publishes bare names (glm-5). Requiring a slash would have rejected its entire catalog, so id validation is per-router on both sides (ROUTER_MODEL_ID_RE in routes/user.ts, ROUTER_MODEL_ID in RouterSettingsSection.tsx) and the shared resolveModel regex has a matching branch.

Validation

Against main @ 5b441de:

• backend: tsc --noEmit clean; vitest run - 699 passed, 24 skipped (baseline on main: 683 passed; +16 new tests covering the adapter, model ids, the catalog route, the per-router id shapes and the selection gate).
• frontend: tsc --noEmit clean; eslint 0 errors; vitest run - 466 passed (baseline: 461).
• word-addin: typecheck:app and typecheck:e2e clean; the cross-package catalog-parity and composer-availability drift guards extended to the new router and passing.

Three frontend files (mikeApi, useSelectedModel, ChatInput.modelSelection) fail identically on unmodified main in my sandbox - Blob.text is not a function and window.localStorage undefined, a local jsdom/env issue rather than a code change. Baseline is 12 failures, this branch is 14; the two extra are new useSelectedModel tests blocked by that same missing localStorage, and CI should run them.