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.

tsconfig.json

@@ -0,0 +1,16 @@
+{
+  "compilerOptions": {
+    "target": "ESNext",
+    "module": "ESNext",
+    "moduleResolution": "bundler",
+    "jsx": "react-jsx",
+    "jsxImportSource": "@opentui/solid",
+    "strict": true,
+    "noEmit": true,
+    "noUncheckedIndexedAccess": true,
+    "skipLibCheck": true,
+    "verbatimModuleSyntax": true,
+    "types": ["node"]
+  },
+  "include": ["src", "test"]
+}