herdr: distribute the SKILL.md file

By distributing the SKILL.md file, we can configure LLM tooling to pick
up the skill from the filesystem instead of either manually keeping
a local copy in sync or doing a remote fetch.

An example from how this could be used from home-manager:

	programs.opencode.settings.skills.paths = [
	  "${pkgs.herdr}/share/herdr/skills"
	];

There doesn't seem to be a convention within nixpkgs of how to
distribute skills, so I've taken the liberty of using the $out/share
directory.

Signed-off-by: Tristan Partin <tristan@partin.io>

pkgs/by-name/he/herdr/package.nix

@@ -55,7 +55,10 @@ rustPlatform.buildRustPackage (finalAttrs: {
     chmod -R u+w "$ZIG_GLOBAL_CACHE_DIR/p"
   '';
 
-  postInstall = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+  postInstall = ''
+    install -Dm444 SKILL.md -t $out/share/herdr/skills/herdr
+  ''
+  + lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
     installShellCompletion --cmd herdr \
       --bash <("$out/bin/herdr" completion bash) \
       --fish <("$out/bin/herdr" completion fish) \