chore: update readme (#98)

gigacode/README.md

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+<p align="center">
+  <img src="../.github/media/gigacode-header.jpeg" alt="Gigacode. Use OpenCode's UI with any coding agent." />
+</p>
+
+<h3 align="center">Supports Claude Code, Codex, and Amp.</h3>
+
+<p align="center">
+    <i>This is <u>not</u> a fork (and never will be).<br/>It's powered by <a href="https://sandboxagent.dev">Sandbox Agent SDK</a>'s wizardry.<br/>Experimental & just for fun.</i>
+</p>
+
+<p align="center">
+    <a href="https://github.com/rivet-dev/sandbox-agent/issues">Issues</a> — <a href="https://rivet.dev/discord">Discord</a> — <a href="https://sandboxagent.dev/docs/opencode-compatibility#endpoint-coverage">Supported OpenCode Features</a>
+</p>
+
+
+## How It Works
+
+```
+┌─ Gigacode ────────────────────────────────────────────────────────┐
+│ ┌─────────────────┐    ┌─────────────────┐    ┌─────────────────┐ │
+│ │  OpenCode TUI   │───▶│  Sandbox Agent  │───▶│  Claude Code /  │ │
+│ │                 │    │                 │    │   Codex / Amp   │ │
+│ └─────────────────┘    └─────────────────┘    └─────────────────┘ │
+└───────────────────────────────────────────────────────────────────┘
+```
+
+- [Sandbox Agent SDK](https://sandboxagent.dev) provides a universal HTTP API for controlling Claude Code, Codex, and Amp
+- Sandbox Agent SDK exposes an [OpenCode-compatible endpoint](https://sandboxagent.dev/docs/opencode-compatibility) so OpenCode can talk to any agent
+- OpenCode connects to Sandbox Agent SDK via [`attach`](https://opencode.ai/docs/cli/#attach)
+
+## OpenCode Models vs Gigacode Agents
+
+- **OpenCode** supports **switching between inference providers** (Anthropic, OpenAI, etc.). This is OpenCode talking directly to the models with its own tools, system prompts, and agentic loop.
+- **Gigacode** automates other coding agent harnesses, so it's using the **exact same logic that you would if you ran Claude Code**, Codex, or Amp natively.
+
+```
+OpenCode (native):   Model → OpenCode's tool loop → result
+Gigacode:            Model → Claude Code / Codex / Amp CLI → result
+```
+
+This means you get each agent's specialized capabilities (such as Claude Code's `Read`/`Write`/`Bash` tools, Codex's sandboxed execution, and Amp's permission rules) rather than a single tool loop with different models behind it.
+
+## Install
+
+**macOS / Linux / WSL (Recommended)**
+
+```bash
+curl -fsSL https://releases.rivet.dev/sandbox-agent/latest/gigacode-install.sh | sh
+```
+
+**npm i -g**
+
+```bash
+npm install -g gigacode
+gigacode --help
+```
+
+**bun add -g**
+
+```bash
+bun add -g gigacode
+# Allow Bun to run postinstall scripts for native binaries.
+bun pm -g trust gigacode-linux-x64 gigacode-linux-arm64 gigacode-darwin-arm64 gigacode-darwin-x64 gigacode-win32-x64
+gigacode --help
+```
+
+**npx**
+
+```bash
+npx gigacode --help
+```
+
+**bunx**
+
+```bash
+bunx gigacode --help
+```
+
+> **Note:** Windows is unsupported. Please use [WSL](https://learn.microsoft.com/en-us/windows/wsl/install).
+
+## Usage
+
+**TUI**
+
+Launch the OpenCode TUI with any coding agent:
+
+```bash
+gigacode
+```
+
+**Web UI**
+
+Use the [OpenCode Web UI](https://sandboxagent.dev/docs/opencode-compatibility) to control any coding agent from the browser.
+
+**OpenCode SDK**
+
+Use the [`@opencode-ai/sdk`](https://sandboxagent.dev/docs/opencode-compatibility) to programmatically control any coding agent.