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

CONTRIBUTING should now solely be about the contribution *process*
while HACKING goes into the technical details

This adds two explicit `zig build` steps: `run-valgrind` and
`test-valgrind` to run the Ghostty exe or tests under Valgrind,
respectively.

This simplifies the manual Valgrind calls in a few ways:

1. It automatically sets the CPU to baseline, which is a frequent and
   requirement for Valgrind on newer CPUs, and generally safe.

2. It sets up the rather complicated set of flags to call Valgrind with,
   importantly setting up our suppressions.

3. It enables pairing it with the typical and comfortable workflow of
   specifying extra args (with `--`) or flags like `-Dtest-filter` for
   tests.

For some reason it pointed to the Contributor's Guide instead...

This is my third (!) attempt at implementing localization support.
By leveraging GTK builder to do most of the `gettext` calls, I
can avoid the whole mess about missing symbols on non-glibc platforms.

Added some documentation too for contributors and translators,
just for good measure.