ci/treefmt: add yamlfmt

Most workflow files are already well formatted, but to make it easier to
keep it that way, we can add yamlfmt.

I personally have a preference for non-indented arrays for YAML, but
wanted to avoid bigger diffs here - the status-quo clearly are indented
arrays.

Some changes are made manually to the get-merge-commit action and the
issue templates. Those would otherwise make yamlfmt misbehave on those.

.github/ISSUE_TEMPLATE/01_bug_report.yml

@@ -101,7 +101,7 @@ body:
       label: "Notify maintainers"
       description: |
         Please mention the people who are in the **Maintainers** list of the offending package. This is done by by searching for the package on the [NixOS Package Search](https://search.nixos.org/packages) and mentioning the people listed under **Maintainers** by prefixing their GitHub usernames with an '@' character. Please add the mentions above the `---` characters in the template below.
-      value: |
+      value: |2
 
 
         ---

.github/ISSUE_TEMPLATE/02_bug_report_darwin.yml

@@ -115,7 +115,7 @@ body:
         Please mention the people who are in the **Maintainers** list of the offending package. This is done by by searching for the package on the [NixOS Package Search](https://search.nixos.org/packages) and mentioning the people listed under **Maintainers** by prefixing their GitHub usernames with an '@' character. Please add the mentions above the `---` characters in the template below.
 
         If this issue is related to the Darwin packaging architecture as a whole, or is related to the core Darwin frameworks, consider mentioning the `@NixOS/darwin-core` team.
-      value: |
+      value: |2
 
 
         ---

.github/ISSUE_TEMPLATE/03_bug_report_nixos.yml

@@ -105,7 +105,7 @@ body:
         Please note that the maintainer attribute name does not always match the maintainer's GitHub username. If that occurs, try looking in [`maintainers/maintainer-list.nix`](https://github.com/NixOS/nixpkgs/blob/master/maintainers/maintainer-list.nix) for the maintainer attribute name, and checking if the maintainer has a listed GitHub username.
 
         If in doubt, check `git blame` for whoever last touched the module, or check the associated package's maintainers. Please add the mentions above the `---` characters.
-      value: |
+      value: |2
 
 
         ---

.github/ISSUE_TEMPLATE/04_build_failure.yml

@@ -111,7 +111,7 @@ body:
       label: "Notify maintainers"
       description: |
         Please mention the people who are in the **Maintainers** list of the offending package. This is done by by searching for the package on the [NixOS Package Search](https://search.nixos.org/packages) and mentioning the people listed under **Maintainers** by prefixing their GitHub usernames with an '@' character. Please add the mentions above the `---` characters in the template below.
-      value: |
+      value: |2
 
 
         ---

.github/ISSUE_TEMPLATE/05_update_request.yml

@@ -86,7 +86,7 @@ body:
       label: "Notify maintainers"
       description: |
         Please mention the people who are in the **Maintainers** list of the offending package. This is done by by searching for the package on the [NixOS Package Search](https://search.nixos.org/packages) and mentioning the people listed under **Maintainers** by prefixing their GitHub usernames with an '@' character. Please add the mentions above the `---` characters in the template below.
-      value: |
+      value: |2
 
 
         ---

.github/ISSUE_TEMPLATE/06_module_request.yml

@@ -61,7 +61,7 @@ body:
       label: "Notify maintainers"
       description: |
         Please mention the people who are in the **Maintainers** list of the offending package. This is done by by searching for the package on the [NixOS Package Search](https://search.nixos.org/packages) and mentioning the people listed under **Maintainers** by prefixing their GitHub usernames with an '@' character. Please add the mentions above the `---` characters in the template below.
-      value: |
+      value: |2
 
 
         ---

.github/ISSUE_TEMPLATE/07_backport_request.yml

@@ -66,7 +66,7 @@ body:
       label: "Notify maintainers"
       description: |
         Please mention the people who are in the **Maintainers** list of the offending package. This is done by by searching for the package on the [NixOS Package Search](https://search.nixos.org/packages) and mentioning the people listed under **Maintainers** by prefixing their GitHub usernames with an '@' character. Please add the mentions above the `---` characters in the template below.
-      value: |
+      value: |2
 
 
         ---

.github/ISSUE_TEMPLATE/08_documentation_request.yml

@@ -48,7 +48,7 @@ body:
       label: "Notify maintainers"
       description: |
         Please mention the people who are in the **Maintainers** list of the offending package. This is done by by searching for the package on the [NixOS Package Search](https://search.nixos.org/packages) and mentioning the people listed under **Maintainers** by prefixing their GitHub usernames with an '@' character. Please add the mentions above the `---` characters in the template below.
-      value: |
+      value: |2
 
 
         ---

.github/ISSUE_TEMPLATE/09_unreproducible_package.yml

@@ -120,7 +120,7 @@ body:
       label: "Notify maintainers"
       description: |
         Please mention the people who are in the **Maintainers** list of the offending package. This is done by by searching for the package on the [NixOS Package Search](https://search.nixos.org/packages) and mentioning the people listed under **Maintainers** by prefixing their GitHub usernames with an '@' character. Please add the mentions above the `---` characters in the template below.
-      value: |
+      value: |2
 
 
         ---

.github/actions/get-merge-commit/action.yml

@@ -72,10 +72,10 @@ runs:
           }
           throw new Error("Not retrying anymore. It's likely that GitHub is having internal issues: check https://www.githubstatus.com.")
 
+    - if: inputs.merged-as-untrusted && steps.commits.outputs.mergedSha
       # Would be great to do the checkouts in git worktrees of the existing spare checkout instead,
       # but Nix is broken with them:
       # https://github.com/NixOS/nix/issues/6073
-    - if: inputs.merged-as-untrusted && steps.commits.outputs.mergedSha
       uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       with:
         ref: ${{ steps.commits.outputs.mergedSha }}

ci/default.nix

@@ -52,6 +52,21 @@ let
         # See https://github.com/NixOS/nixfmt
         programs.nixfmt.enable = true;
 
+        programs.yamlfmt = {
+          enable = true;
+          settings.formatter = {
+            retain_line_breaks = true;
+          };
+        };
+        settings.formatter.yamlfmt.excludes = [
+          # Breaks helm templating
+          "nixos/tests/k3s/k3s-test-chart/templates/*"
+          # Aligns comments with whitespace
+          "pkgs/development/haskell-modules/configuration-hackage2nix/main.yaml"
+          # TODO: Fix formatting for auto-generated file
+          "pkgs/development/haskell-modules/configuration-hackage2nix/transitive-broken.yaml"
+        ];
+
         settings.formatter.editorconfig-checker = {
           command = "${pkgs.lib.getExe pkgs.editorconfig-checker}";
           options = [ "-disable-indent-size" ];