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/format.test.ts

@@ -0,0 +1,49 @@
+import { describe, expect, it } from "vitest"
+import { formatBalanceError, formatCredits, readFoldOpen } from "../src/format"
+
+describe("formatCredits", () => {
+  it("formats with two decimals", () => {
+    expect(formatCredits(5.309411174)).toBe("5.31")
+  })
+
+  it("formats zero", () => {
+    expect(formatCredits(0)).toBe("0.00")
+  })
+
+  it("returns a dash for non-finite input", () => {
+    expect(formatCredits(Number.NaN)).toBe("-")
+  })
+})
+
+describe("formatBalanceError", () => {
+  it("renders each error kind", () => {
+    expect(formatBalanceError({ kind: "unauthorized" })).toContain("invalid")
+    expect(formatBalanceError({ kind: "network" })).toContain("network")
+    expect(formatBalanceError({ kind: "missing-header", status: 400 })).toContain("400")
+    expect(formatBalanceError({ kind: "no-credential", reason: "missing" })).toContain("GreenPT")
+  })
+})
+
+describe("readFoldOpen", () => {
+  it("defaults to open when the KV store is absent", () => {
+    expect(readFoldOpen(undefined)).toBe(true)
+  })
+
+  it("reads a persisted boolean", () => {
+    expect(readFoldOpen({ get: () => false })).toBe(false)
+  })
+
+  it("falls back when the stored value is not a boolean", () => {
+    expect(readFoldOpen({ get: () => "nope" })).toBe(true)
+  })
+
+  it("survives a throwing KV store", () => {
+    expect(
+      readFoldOpen({
+        get: () => {
+          throw new Error("kv down")
+        },
+      }),
+    ).toBe(true)
+  })
+})