Document flake outputs and usage

- Describe exported packages, apps, and checks
- Add build, run, and flake inspection examples

README.md

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+# t3code-flake
+
+Standalone Nix packaging for [`pingdotgg/t3code`](https://github.com/pingdotgg/t3code).
+
+This repository does not rely on upstream Nix packaging. It fetches the upstream source as a non-flake input and builds local package expressions around that source tree.
+
+## What It Exports
+
+For each supported system, the flake exports:
+
+- `packages.<system>.t3code-server`
+- `packages.<system>.t3code-desktop`
+- `packages.<system>.default`
+- `apps.<system>.t3code-server`
+- `apps.<system>.t3code-desktop`
+- `apps.<system>.default`
+- `checks.<system>.t3code-server`
+- `checks.<system>.t3code-desktop`
+
+Current `default` points to the desktop package/app.
+
+## Usage
+
+Build the server package:
+
+```bash
+nix build .#t3code-server
+```
+
+Build the desktop package:
+
+```bash
+nix build .#t3code-desktop
+```
+
+Run the server app:
+
+```bash
+nix run .#t3code-server
+```
+
+Run the desktop app:
+
+```bash
+nix run .#t3code-desktop
+```
+
+Inspect the flake surface:
+
+```bash
+nix flake show --all-systems
+```
+
+## Layout
+
+```text
+.
+├── flake.nix
+├── flake.lock
+├── README.md
+└── packages
+    ├── common.nix
+    ├── desktop
+    │   ├── build-nix-desktop-package.mjs
+    │   ├── default.nix
+    │   └── desktop-nix-autoupdate.patch
+    └── server
+        └── default.nix
+```
+
+## Notes
+
+- `packages/common.nix` contains the shared Bun workspace and `node-pty` setup used by both packages.
+- `packages/desktop/build-nix-desktop-package.mjs` is local packaging glue carried by this repo.
+- `packages/desktop/desktop-nix-autoupdate.patch` is a downstream patch for desktop auto-update messaging in package-managed installs.
+- Upstream source is pinned in `flake.lock` via the `t3code` input.