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.

package.json

@@ -0,0 +1,36 @@
+{
+  "name": "opencode-greenpt-usage",
+  "version": "0.1.0",
+  "description": "OpenCode TUI plugin that shows the remaining GreenPT API credit balance in the sidebar.",
+  "type": "module",
+  "license": "AGPL-3.0-or-later",
+  "keywords": [
+    "opencode",
+    "opencode-plugin",
+    "tui",
+    "greenpt",
+    "credits",
+    "balance"
+  ],
+  "engines": {
+    "opencode": ">=1.18.0"
+  },
+  "scripts": {
+    "test": "vitest run",
+    "test:integration": "GREENPT_INTEGRATION=1 vitest run test/integration.greenpt.test.ts",
+    "typecheck": "tsc --noEmit"
+  },
+  "peerDependencies": {
+    "@opencode-ai/plugin": "*",
+    "@opentui/solid": "*",
+    "solid-js": "*"
+  },
+  "devDependencies": {
+    "@opencode-ai/plugin": "^1.18.33",
+    "@opentui/solid": "^0.5.12",
+    "@types/node": "^24.0.0",
+    "solid-js": "^1.9.0",
+    "typescript": "^5.6.0",
+    "vitest": "^3.0.0"
+  }
+}