ai: add gh-issue command to help diagnose GitHub issues

This enables agents (namely Amp) to use `/gh-issue <number/url>` to 
begin diagnosing a GitHub issue, explaining the problem, and suggesting
a plan of action. This action explicitly prompts the AI to not write
code.

I've used this manually for months with good results, so now I'm
formalizing it in the repo for other contributors.

Example diagnosing #8523:

https://ampcode.com/threads/T-3e26e8cc-83d1-4e3c-9b5e-02d9111909a7

nix/devShell.nix

@@ -3,6 +3,7 @@
   lib,
   stdenv,
   bashInteractive,
+  nushell,
   appstream,
   flatpak-builder,
   gdb,
@@ -124,6 +125,9 @@ in
         # CI
         uv
 
+        # Scripting
+        nushell
+
         # We need these GTK-related deps on all platform so we can build
         # dist tarballs.
         blueprint-compiler