new: [opencode-greenpt-usage] add GreenPT credit balance plugin

Show the remaining GreenPT API credit balance in the OpenCode TUI
sidebar. The key is resolved from the ``apiKey`` option, the
``GREENPT_API_KEY`` environment variable, or the active OpenCode
``auth.json`` (provider ``greenpt``); the section is hidden when no key
is present, so the plugin stays portable across machines and profiles.

The balance is read from the ``x-credits-remaining`` header returned by
a free ``HEAD /v1/chat/completions`` probe (HTTP 400 before any credit
is consumed), which avoids a billable call per refresh. Tests cover the
credential resolution, the header parsing and the live API.

Distribute via git clone; there is no npm publish.

test/integration.greenpt.test.ts

@@ -0,0 +1,24 @@
+import { describe, expect, it } from "vitest"
+import { fetchBalance } from "../src/balance"
+import { DEFAULT_BASE_URL } from "../src/store"
+
+const apiKey = process.env.GREENPT_API_KEY
+const enabled = process.env.GREENPT_INTEGRATION === "1" && typeof apiKey === "string" && apiKey.length > 0
+
+const suite = enabled ? describe : describe.skip
+
+suite("GreenPT live API", () => {
+  it("returns a finite balance from the free HEAD probe", async () => {
+    const result = await fetchBalance(DEFAULT_BASE_URL, apiKey as string)
+    if ("kind" in result) {
+      throw new Error(`expected a balance, got error: ${result.kind}`)
+    }
+    expect(Number.isFinite(result.credits)).toBe(true)
+    expect(result.credits).toBeGreaterThanOrEqual(0)
+  })
+
+  it("reports unauthorized for an invalid key", async () => {
+    const result = await fetchBalance(DEFAULT_BASE_URL, "sk-definitely-invalid")
+    expect(result).toEqual({ kind: "unauthorized" })
+  })
+})