docs: add project readme and usage attribution

AGENTS.md

@@ -12,7 +12,7 @@
 - `render.ts` — Width allocation, ANSI-aware truncation, dim text rendering.
 - `modules/basic.ts` — Directory, context, model, thinking, and cost modules.
 - `modules/usage.ts` — Usage bars/text modules and color gradient rendering.
-- `usage/` — Provider usage fetchers copied/adapted from `oh-my-pi`.
+- `usage/` — Provider usage fetchers adapted from `oh-my-pi`: https://github.com/can1357/oh-my-pi/tree/main/packages/ai/src/usage
 - `providers/openai-codex/constants.ts`, `usage.ts`, `utils.ts` — Local shims/support for copied usage code.
 
 ## Conventions

README.md

@@ -0,0 +1,52 @@
+# pi-statusbar
+
+A [pi](https://github.com/badlogic/pi-mono/tree/main/packages/coding-agent) extension that replaces the built-in footer with a configurable two-row statusbar.
+
+## Features
+
+- Directory, git branch, session, context, model, thinking, and cost modules
+- Account usage modules for Claude/Anthropic, Codex/ChatGPT, and Z.ai
+- Configurable rows and module placement in `config.ts`
+- ANSI-aware rendering and truncation for narrow terminals
+- Continuous green → yellow → red usage bar gradient
+
+## Usage
+
+Add this extension to pi via `package.json`:
+
+```json
+{
+  "pi": {
+    "extensions": ["./index.ts"]
+  }
+}
+```
+
+Reload pi after changes with `/reload`.
+
+Refresh account usage manually with:
+
+```text
+/refresh-usage
+```
+
+## Configuration
+
+Edit `config.ts` to change the statusbar layout. Usage modules can be composed with options such as:
+
+```ts
+{ type: "usage", window: "current", style: "line", grow: 1 }
+{ type: "usage", window: "week", style: "text", parts: ["percent", "time"] }
+```
+
+## Development
+
+```sh
+npm install
+npm run lint
+npm run typecheck
+```
+
+## Attribution
+
+Provider usage fetchers in `usage/` are adapted from [`oh-my-pi`](https://github.com/can1357/oh-my-pi/tree/main/packages/ai/src/usage).