Adds the Metal Toolchain as a required Xcode component for building
Ghostty. Also updates the notes about Xcode 26 now that it and Tahoe are
out of Beta.
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
Also did some copy-editing and removed some things that haven't been
present in the codebase for a while (I thought we nuked conformance
stuff in 1.1 already...?)
