macos: lint Swift files using SwiftLint

SwiftLint <https://realm.github.io/SwiftLint/> is both a linter and
formatting. It's a popular way to spot issues and enforce a consistent
style.

Our SwiftLint configuration lives in macos/.swiftlint.yml, where is is
automatically discovered. It's very configurable, and I made an initial
pass as some basic, weakly-opinionated rules. The "TODO" section lists
rules that currently have violations but can be easily (auto)fixed in
follow-up commits.

Our integration is CLI-based. Similar to our other support tools, we
expect developers to install `swiftlint` via nix or e.g. Homebrew.
This is documented in HACKING.md.

We also have an optional Xcode integration, for in-editor feedback. When
`swiftlint` is available, it's run as a script-based Build Phase.

SwiftLint supports an auto-fix mode (--fix). Agents are aware of this
via AGENTS.md.

The rules are enforced using a (nix-based) CI job.

nix/devShell.nix

@@ -66,6 +66,7 @@
   poop,
   typos,
   shellcheck,
+  swiftlint,
   uv,
   wayland,
   wayland-scanner,
@@ -198,6 +199,9 @@ in
 
         # for benchmarking
         poop
+      ]
+      ++ lib.optionals stdenv.hostPlatform.isDarwin [
+        swiftlint
       ];
 
     # This should be set onto the rpath of the ghostty binary if you want