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.

AGENTS.md

@@ -0,0 +1,30 @@
+# AGENTS.md
+
+## Project
+
+`opencode-greenpt-usage` — an OpenCode **TUI** plugin (Solid + OpenTUI) that
+shows the remaining GreenPT API credit balance in the sidebar. Distributed by
+`git clone`; there is no npm publish.
+
+## Commands
+
+```bash
+npm install              # install dev dependencies
+npm test                 # unit tests (vitest)
+npm run typecheck        # tsc --noEmit
+GREENPT_API_KEY=... npm run test:integration   # live API test
+```
+
+## Conventions
+
+- The plugin entry is the `default export { id, tui }` in `src/index.tsx`.
+  Types come from `@opencode-ai/plugin/tui` as **type-only** imports; the
+  host provides `solid-js` and `@opentui/solid` at runtime.
+- The section is registered on the `sidebar_content` slot and is hidden when
+  no GreenPT credential is present (`showWhenUnavailable` opts into a hint).
+- Balance probing must stay **free**: `HEAD /v1/chat/completions` returns
+  `400` plus `x-credits-remaining`. Never replace it with a billable call.
+- Credential resolution order: `apiKey` option, `GREENPT_API_KEY`, active
+  OpenCode `auth.json` (`greenpt`, `type: "api"`). No profile-switching
+  command may leak into this logic.
+- No code comments unless functionally required.