Add HLint workflow.

.github/workflows/hlint.yml

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+name: HLint
+on: [push, pull_request]
+
+jobs:
+  hlint:
+    runs-on: ubuntu-latest
+    permissions:
+      # Needed to upload results to GitHub code scanning.
+      security-events: write
+    steps:
+      - uses: actions/checkout@d632683dd7b4114ad314bca15554477dd762a938 # v4.2.0
+      - uses: haskell-actions/hlint-scan@c81f3176108e60bbce2d5a223e9297a8406deb92 # v1.1.0

.hlint.yaml

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+# HLint configuration file
+# https://github.com/ndmitchell/hlint
+##########################
+
+
+# Ignore generated files.
+- ignore: {within: Proto.Google.Protobuf.Descriptor}
+- ignore: {within: Proto.Google.Protobuf.Descriptor_Fields}
+- ignore: {within: Proto.Google.Protobuf.Compiler.Plugin}
+- ignore: {within: Proto.Google.Protobuf.Compiler.Plugin_Fields}
+
+
+# Warnings currently triggered by existing code.
+- ignore: {name: "Avoid lambda"} # 1 hint
+- ignore: {name: "Eta reduce"} # 1 hint
+- ignore: {name: "Fuse foldr/map"} # 1 hint
+- ignore: {name: "Move brackets to avoid $"} # 8 hints
+- ignore: {name: "Redundant $"} # 23 hints
+- ignore: {name: "Redundant ^."} # 2 hints
+- ignore: {name: "Redundant bracket"} # 13 hints
+- ignore: {name: "Unused LANGUAGE pragma"} # 11 hints
+- ignore: {name: "Use :"} # 2 hints
+- ignore: {name: "Use <$"} # 1 hint
+- ignore: {name: "Use <$>"} # 1 hint
+- ignore: {name: "Use ?~"} # 5 hints
+- ignore: {name: "Use camelCase"} # 3 hints
+- ignore: {name: "Use concatMap"} # 1 hint
+- ignore: {name: "Use fewer imports"} # 2 hints
+- ignore: {name: "Use fmap"} # 10 hints
+- ignore: {name: "Use lambda-case"} # 3 hints
+- ignore: {name: "Use maybe"} # 1 hint
+- ignore: {name: "Use newtype instead of data"} # 1 hint
+- ignore: {name: "Use replicate"} # 1 hint
+- ignore: {name: "Use unless"} # 1 hint