docs(ghostty): add libghostty fork maintenance rules

[nek0der]

Summary

Adds docs/libghostty-fork.md, a standing document for how we maintain the
libghostty fork: why it exists, what a patch must carry, how we rebase, and
how we retire one. Brings the existing fork patch up to those rules by
recording its class and retirement condition in its commit message. No
tracked source changes — the submodule bump rewrites a commit message and
leaves the tree byte-identical.

Related issue

Skip.

How verified

• git check-ignore — docs/libghostty-fork.md is not ignored; local notes
  directly under docs/ still are.
• Every command in the Health section runs against this checkout and returns
  the expected values: one patch carried, 133 commits behind upstream.
• git rev-parse — the submodule's new tip has the same tree as the old one,
  so no build output can change.
• The state pinned before the rewrite stays reachable through the
  pin/2026-09-06 tag, pushed to the fork.
• make lint — Rust and Swift checks pass with zero violations.
• make test — Rust, XCTest, and Swift Testing suites all pass.

Screenshots / video

Skip.

Checklist

• make lint clean locally
• make test passes
• make review-core passes (its own CI job; make test does not cover it)
• Tests added or updated where applicable
• User-facing strings added to Localizable.xcstrings with both en and ja
• Verified manually on macOS 26 (Tahoe) — see "How verified" above
• Code comments are in English