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/credential.test.ts

@@ -0,0 +1,93 @@
+import { afterEach, describe, expect, it } from "vitest"
+import { mkdtempSync, mkdirSync, rmSync, writeFileSync } from "node:fs"
+import { tmpdir } from "node:os"
+import { join } from "node:path"
+import { credentialSearchDirs, resolveGreenptKey } from "../src/credential"
+
+const created: string[] = []
+
+function tempDir(): string {
+  const dir = mkdtempSync(join(tmpdir(), "greenpt-cred-"))
+  created.push(dir)
+  return dir
+}
+
+function writeAuth(dir: string, content: unknown): void {
+  writeFileSync(join(dir, "auth.json"), JSON.stringify(content))
+}
+
+afterEach(() => {
+  while (created.length > 0) {
+    const dir = created.pop()
+    if (dir) rmSync(dir, { recursive: true, force: true })
+  }
+})
+
+describe("resolveGreenptKey", () => {
+  it("prefers the explicit apiKey option", () => {
+    const dir = tempDir()
+    writeAuth(dir, { greenpt: { type: "api", key: "from-auth" } })
+    const result = resolveGreenptKey({ apiKey: "from-option" }, dir, { GREENPT_API_KEY: "from-env" })
+    expect(result).toEqual({ ok: true, key: "from-option" })
+  })
+
+  it("prefers the environment variable over the auth store", () => {
+    const dir = tempDir()
+    writeAuth(dir, { greenpt: { type: "api", key: "from-auth" } })
+    const result = resolveGreenptKey({}, dir, { GREENPT_API_KEY: "from-env" })
+    expect(result).toEqual({ ok: true, key: "from-env" })
+  })
+
+  it("reads the active auth store when nothing else is set", () => {
+    const dir = tempDir()
+    writeAuth(dir, { greenpt: { type: "api", key: "from-auth" } })
+    const result = resolveGreenptKey({}, dir, {})
+    expect(result).toEqual({ ok: true, key: "from-auth" })
+  })
+
+  it("ignores OAuth entries", () => {
+    const dir = tempDir()
+    writeAuth(dir, { greenpt: { type: "oauth", access: "token" } })
+    const result = resolveGreenptKey({}, dir, {})
+    expect(result).toEqual({ ok: false, reason: "missing" })
+  })
+
+  it("ignores empty keys", () => {
+    const dir = tempDir()
+    writeAuth(dir, { greenpt: { type: "api", key: "" } })
+    const result = resolveGreenptKey({}, dir, {})
+    expect(result).toEqual({ ok: false, reason: "missing" })
+  })
+
+  it("returns missing when no credential is available", () => {
+    const dir = tempDir()
+    const result = resolveGreenptKey({}, dir, {})
+    expect(result).toEqual({ ok: false, reason: "missing" })
+  })
+
+  it("falls back to XDG data home", () => {
+    const xdg = tempDir()
+    const stateDir = join(xdg, "opencode")
+    mkdirSync(stateDir)
+    writeAuth(stateDir, { greenpt: { type: "api", key: "from-xdg" } })
+    const result = resolveGreenptKey({}, undefined, { XDG_DATA_HOME: xdg })
+    expect(result).toEqual({ ok: true, key: "from-xdg" })
+  })
+})
+
+describe("credentialSearchDirs", () => {
+  it("puts the state dir first and de-duplicates", () => {
+    const dirs = credentialSearchDirs("/state", {
+      XDG_DATA_HOME: "/xdg",
+      OPENCODE_DATA_DIR: "/state",
+    })
+    expect(dirs[0]).toBe("/state")
+    expect(dirs).toContain(join("/xdg", "opencode"))
+    expect(new Set(dirs).size).toBe(dirs.length)
+  })
+
+  it("falls back to ~/.local/share/opencode", () => {
+    const dirs = credentialSearchDirs(undefined, { HOME: "/home/tester" })
+    expect(dirs).toContain(join("/home/tester", ".local", "share", "opencode"))
+  })
+})