feat: add OpenCode Go usage support

[JairajSustained]

Adds OpenCode Go to /usage.

• New provider key opencode-go (detects model.provider opencode-go/opencode)
• fetchOpencodeGoUsage() via GET https://opencode.ai/zen/go/v1/usage with Bearer API key (no OAuth refresh, no cookie scraping)
• Maps rolling (5h) -> session, weekly -> weekly, monthly -> monthly (new optional UsageData fields + Monthly bar in panel)
• Shows Rolling/Weekly/Monthly with resets in panel; Daily/Weekly unchanged for other providers
• Auth check via hasAuth(opencode-go); verified live (5%/27%/13%)
• Tests for detect + fetch + HTTP error; README updated

[iefnaf]

Thanks for the PR — nice work overall! I verified the core implementation and it holds up well:

• ✅ opencode-go is a real provider in pi-ai (id: "opencode-go", name "OpenCode Go", API-key auth), so the auth lookup id is correct — pi stores API-key providers in auth.json under their provider id.
• ✅ The endpoint is real (I confirmed https://opencode.ai/zen/go/v1/usage returns HTTP 401 without auth).
• ✅ All 6 tests pass, and the defensive field fallbacks + readPercentCandidate handle garbage responses safely.

Two issues to fix before merging:

1. The "opencode" alias in detectProvider is incorrect

if (provider === "opencode-go" || provider === "opencode") return "opencode-go";

pi-ai registers two distinct providers: opencode (OpenCode Zen — the BYO-key marketplace, per-token billing) and opencode-go (the Go subscription plan). They have separate credentials (auth.json["opencode"] vs auth.json["opencode-go"]) and separate usage APIs — /zen/go/v1/usage only reports Go subscription quota.

With this alias, a Zen-only user running /usage gets routed down the Go branch, the key lookup under "opencode-go" fails, and they see "No OpenCode Go API key found (connect OpenCode Go to get a key)" — confusing, since they do have a working key, it's just stored under the other id. It's like routing Anthropic Console API users to the Claude Pro subscription panel.

Please drop the alias and match strictly:

if (provider === "opencode-go") return "opencode-go";

(The test using { provider: "opencode", id: "glm-5.2" } would need updating too.) Zen usage/credits would be a separate feature with its own endpoint if we ever want it.

2. Needs rebase — conflicts with master

This is based on the old master. 7b8ddea (add Kimi, drop Anthropic/Gemini/Antigravity) conflicts in all 4 files — ProviderKey, detectProvider, PROVIDER_LABELS, and canShowForProvider all need reworking onto the new provider list. Also note the imports are still @mariozechner/*; coordinate with #2 (the scope migration) for merge order or fold that in during the rebase.

Minor (non-blocking)

• Missing a test for the "unrecognized response shape" branch (e.g. response without usage or non-numeric percents).
• In canShowForProvider, the (auth as AuthData) casts are redundant — AuthData already declares the "opencode-go" field.

The core of the feature is solid — with the alias removed and rebased onto current master, this should be good to go.