docs(readme): document Linux download and install

README.md

@@ -12,6 +12,82 @@ CastCodes is an open-source terminal and code workspace fork. This repository is
 
 The public CastCodes build is local-only by default. It does not include sign-in, hosted telemetry, hosted crash reporting, billing, shared sessions, upstream release feeds, or upstream feedback flows. Feedback links route to this fork's GitHub issue tracker.
 
+## Install on Linux
+
+Prebuilt CastCodes packages for Linux are published as GitHub release assets at
+[github.com/OpenCoven/cast-codes/releases/latest](https://github.com/OpenCoven/cast-codes/releases/latest).
+The OSS channel currently publishes **x86_64** builds in four formats; each
+asset ships with a matching `.sha256` checksum file.
+
+| Distribution | Asset |
+| --- | --- |
+| Debian, Ubuntu, derivatives | `cast-codes_<version>_amd64.deb` |
+| Fedora, RHEL, derivatives | `cast-codes-v<version>-1.x86_64.rpm` |
+| Arch Linux, derivatives | `cast-codes-v<version>-1-x86_64.pkg.tar.zst` |
+| Any glibc-based distro | `CastCodes-x86_64.AppImage` |
+
+A CLI-only variant of each package (`cast-codes-cli-...`) is also published for
+headless installs.
+
+### Verify the download
+
+Download both the package and its `.sha256` file, then compare the checksum:
+
+```bash
+expected="$(awk '{print $1}' cast-codes_<version>_amd64.deb.sha256)"
+actual="$(sha256sum cast-codes_<version>_amd64.deb | awk '{print $1}')"
+test "$actual" = "$expected"
+```
+
+### Debian / Ubuntu
+
+```bash
+sudo apt install ./cast-codes_<version>_amd64.deb
+```
+
+### Fedora / RHEL
+
+```bash
+sudo dnf install ./cast-codes-v<version>-1.x86_64.rpm
+```
+
+### Arch Linux
+
+```bash
+sudo pacman -U cast-codes-v<version>-1-x86_64.pkg.tar.zst
+```
+
+Packages install the binary to `/opt/castcodes/cast-codes/` and register a
+`cast-codes` launcher and `dev.castcodes.CastCodes.desktop` entry.
+
+### AppImage
+
+The AppImage runs without installation on most modern glibc-based distros:
+
+```bash
+chmod +x CastCodes-x86_64.AppImage
+./CastCodes-x86_64.AppImage
+```
+
+If the AppImage fails to start, install runtime dependencies first — see
+[Build from source on Linux](#build-from-source-on-linux) below for the
+relevant packages, or run `./script/linux/install_runtime_deps` from a
+repository checkout.
+
+### Build from source on Linux
+
+From a checkout, install build and runtime dependencies, then run the app:
+
+```bash
+./script/linux/install_runtime_deps
+./script/run
+```
+
+The dependency installer currently provisions Debian-family systems
+(`apt-get`); on other distros, install the equivalent packages by hand — see
+`script/linux/install_build_deps` and `script/linux/install_runtime_deps` for
+the full lists.
+
 ## Build
 
 ```bash