Use fork of mdformat-gfm for Markdown formatting (#9063)

Author: alexreinking

.github/pull_request_template.md

@@ -20,6 +20,5 @@ least be tagged with the `release_notes` label.
 - [ ] Tests added or updated (not required for docs, CI config, or typo fixes)
 - [ ] Documentation updated (if public API changed)
 - [ ] Python bindings updated (if public API changed)
-- [ ] Benchmarks are included here if the change is intended to affect
-      performance.
+- [ ] Benchmarks are included here if the change is intended to affect performance.
 - [ ] Commits include AI attribution where applicable (see Code of Conduct)

.pre-commit-config.yaml

@@ -82,10 +82,11 @@ repos:
       - id: mdformat
         args: [ --number, --wrap=80 ]
         additional_dependencies:
-          - mdformat-gfm
+          # Upstream mdformat-gfm declined to work around GitHub's rendering
+          # quirks. See: https://github.com/hukkin/mdformat-gfm/issues/58
+          - git+https://github.com/halide/mdformat-gfm.git@fce9859
           - mdformat-ruff
           - mdformat-shfmt
-        exclude: ^.github/pull_request_template\.md$  # mdformat-gfm doesn't understand GitHub's checklists.
 
   - repo: https://github.com/astral-sh/ruff-pre-commit
     rev: v0.15.5