This adds a new script we can manually run that downloads all the files
that need to be uploaded to the mirror and updates our build.zig.zon.
The upload still happens manually [by me] but this simplifies the task
greatly.

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

- Builds with Zig 0.15 now (but still works just fine with Zig
  0.14 projects).
- Fixes a double-free if nix-prefetch-git can't be found or errors out
- Adds support for generating Flatpak package metadata natively.