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