macos: lint Swift files using SwiftLint (#10860)

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.

.github/workflows/test.yml

@@ -11,10 +11,25 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  paths:
+    if: github.repository == 'ghostty-org/ghostty'
+    runs-on: namespace-profile-ghostty-xsm
+    outputs:
+      macos: ${{ steps.filter.outputs.macos }}
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: dorny/paths-filter@de90cc6fb38fc0963ad72b210f1f284cd68cea36 # v3.0.2
+        id: filter
+        with:
+          filters: |
+            macos:
+              - 'macos/**'
+
   required:
     name: "Required Checks: Test"
     runs-on: namespace-profile-ghostty-xsm
     needs:
+      - paths
       - build-bench
       - build-dist
       - build-examples
@@ -36,6 +51,7 @@ jobs:
       - test-macos
       - pinact
       - prettier
+      - swiftlint
       - alejandra
       - typos
       - shellcheck
@@ -927,6 +943,28 @@ jobs:
       - name: prettier check
         run: nix develop -c prettier --check .
 
+  swiftlint:
+    if: github.repository == 'ghostty-org/ghostty' && needs.paths.outputs.macos == 'true'
+    runs-on: namespace-profile-ghostty-macos-tahoe
+    needs: paths
+    timeout-minutes: 60
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      # TODO(tahoe): https://github.com/NixOS/nix/issues/13342
+      - uses: DeterminateSystems/nix-installer-action@main
+        with:
+          determinate: true
+      - uses: cachix/cachix-action@3ba601ff5bbb07c7220846facfa2cd81eeee15a1 # v16
+        with:
+          name: ghostty
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+          skipPush: true
+          useDaemon: false # sometimes fails on short jobs
+
+      - name: swiftlint check
+        run: nix develop -c swiftlint lint --strict macos
+
   alejandra:
     if: github.repository == 'ghostty-org/ghostty'
     runs-on: namespace-profile-ghostty-xsm