We may not stick with Doxygen, but it gives us something to start with.

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

GTK dynamically loads librsvg when it needs to convert a SVG-only icon
(or any other SVG-only resource) for display. This PR adds the libraries
that GTK needs so that running programs from within the developer shell
can display those SVG resources.

Add shellcheck to CI pipeline to ensure shell scripts follow best practices
and catch common errors. Fix existing shellcheck warnings in test scripts
to pass the new linting requirements.