enforce 'yamllint' checks (#209)

Contributes to rapidsai/build-planning#305

Proposes enforcing `yamllint` checks here. My primary motivation is to catch correctness issues in `dependencies.yaml` files, like duplicate entries silently resolving to the last one or indentation mistakes leading to filters being ignored.

But this also has some side benefits for consistency, which makes it a bit easier to write automation.

Authors:
  - James Lamb (https://github.com/jameslamb)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: #209

Author: jameslamb

.github/CODEOWNERS

@@ -10,6 +10,7 @@ docs/              @rapidsai/cugraph-doc-codeowners
 
 # CI code owners
 /.github/                @rapidsai/ci-codeowners
+/.yamllint.yaml          @rapidsai/ci-codeowners
 /ci/                     @rapidsai/ci-codeowners
 
 # packaging code owners

.pre-commit-config.yaml

@@ -16,6 +16,15 @@ repos:
     hooks:
       - id: shellcheck
         args: ["--severity=warning"]
+  - repo: https://github.com/adrienverge/yamllint
+    rev: v1.38.0
+    hooks:
+      - id: yamllint
+        additional_dependencies: [pyyaml]
+        exclude: |
+          (?x)^(
+            conda/environments/.*
+          )
   - repo: https://github.com/zizmorcore/zizmor-pre-commit
     rev: v1.24.1
     hooks:

.yamllint.yaml

@@ -0,0 +1,37 @@
+# SPDX-FileCopyrightText: Copyright (c) 2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+extends: default
+
+rules:
+  anchors:
+    forbid-undeclared-aliases: true
+    forbid-duplicated-anchors: true
+    forbid-unused-anchors: true
+  braces:
+    forbid: false
+    min-spaces-inside: 0
+    # allow 1 space for jinja templating in conda recipes
+    max-spaces-inside: 1
+    min-spaces-inside-empty: -1
+    max-spaces-inside-empty: -1
+  brackets: enable
+  colons:
+    max-spaces-before: 0
+    max-spaces-after: 1
+  commas:
+    max-spaces-before: 0
+    min-spaces-after: 1
+    max-spaces-after: 1
+  comments: disable
+  comments-indentation: disable
+  document-end: disable
+  document-start: disable
+  key-duplicates:
+    forbid-duplicated-merge-keys: true
+  line-length: disable
+  truthy:
+    allowed-values: ['false', 'true']
+    # having problematic value in keys is rare... and also
+    # GitHub Actions' choie of 'on:' triggers this check
+    # ref: https://github.com/adrienverge/yamllint/issues/430
+    check-keys: false