docs: state OS compatibility explicitly (Linux-only)

Adds an OS Compatibility section to the README and the Installation
manual page: developed and tested on Arch Linux, macOS and Windows
unsupported, with the specific Linux-only calls that back that claim
(systemd-inhibit, zramctl/swapon, sbctl, wl-copy/xclip with no
pbcopy/clip.exe fallback, GNU-only stat/numfmt flags) and the baseline
packages (git, gpg, tar, coreutils) assumed present rather than tracked
by fish-deps.

Adds os/os-compatibility/operating-system/compatibility/linux/macos/
mac/windows/wsl keywords to fish-config.index so `help config os` and
friends jump straight to it.

docs/fish-config.index

@@ -360,6 +360,15 @@ fish-plugins-manifest=## fish_plugins Manifest
 # ── Section 10: Installation ────────────────────────────────────
 installation=# 10. INSTALLATION
 install=# 10. INSTALLATION
+os=## OS Compatibility
+os-compatibility=## OS Compatibility
+operating-system=## OS Compatibility
+compatibility=## OS Compatibility
+linux=## OS Compatibility
+macos=## OS Compatibility
+mac=## OS Compatibility
+windows=## OS Compatibility
+wsl=## OS Compatibility
 sentinel=## Return Sentinel
 updating=## Updating
 

docs/manual/10-installation.md

@@ -6,6 +6,13 @@ sidebar:
 helpKeywords:
 - installation
 - install
+- os
+- operating system
+- compatibility
+- linux
+- macos
+- windows
+- wsl
 ---
 
 This configuration is managed as a git repository. To deploy on a new machine:
@@ -17,6 +24,41 @@ Then open a new Fish shell. Fisher installs automatically on first launch
 and the Catppuccin Mocha theme is applied. All other plugin functionality is
 bundled directly with this config and requires no additional installation.
 
+## OS Compatibility
+
+This is a **Linux-only** configuration. It is developed and tested on an
+Arch Linux system; `fish-deps` also detects `apt`, `dnf`, `zypper`, and
+`yum` for broader distro support, but coverage outside Arch is thinner.
+
+IMPORTANT: A number of functions call Linux-specific subsystems directly, with
+no fallback:
+- `systemd-inhibit` (`wake-lock`)
+- `zramctl` / `swapon` (`swapstat`)
+- `sbctl` and UEFI Secure Boot state (`sbver`)
+- `wl-copy` / `xclip` for clipboard access (`y`, `p`, `paste`, `hist`) —
+  Wayland or X11 only, no `pbcopy`/`pbpaste` fallback
+- GNU coreutils flags such as `stat -c` and `numfmt` (`sudo-toggle`,
+  `dng2avif`), which differ or don't exist under a BSD userland
+
+**macOS** is not supported. `_fish_deps_detect_pm` does check for `brew`, but
+that alone does not make the functions above work — they have no macOS
+equivalent path today.
+
+**Windows** is not supported. Fish itself has no native Windows build;
+upstream's own "Windows" install docs are Cygwin/WSL workarounds, not a real
+port. This config is not tested under WSL either. WSL2 runs a real Linux
+kernel and can run `systemd`, so basic shell use may work, but `zramctl`,
+`sbctl`, and Secure Boot state are meaningless inside a VM, and clipboard
+integration would need a WSL-specific path (`clip.exe`, `win32yank`) that
+does not exist here.
+
+**Assumed present on any Linux system this runs on:** `git`, `gpg`, `tar`,
+and GNU coreutils (for `stat`, `date`, `numfmt`). These are not tracked by
+`fish-deps` — see the [Dependency Catalog](/06-dependency-catalog/) — because
+they are base-system utilities, not opt-in software with an install journey
+to manage. A system missing any of them is missing basic Linux tooling, not
+a `fish-deps` gap.
+
 ## Return Sentinel
 
 `config.fish` ends with a return sentinel guard. Any lines appended after it by