feat(ci): add comprehensive PR checks workflow

Added PR validation workflow with 7 essential checks:

1. helm-docs diff check - ensures README.md is regenerated
2. helm-unittest - runs all 603 tests
3. helm lint - includes values.schema.json validation
4. helm template - validates manifest generation
5. chart-testing (ct lint) - YAML lint, version check, best practices
6. kubeconform - validates K8s manifests against versions 1.29-1.31
7. markdownlint - validates markdown documentation

Configuration files:
- ct.yaml - chart-testing settings
- .yamllint.yaml - YAML linting rules
- .markdownlint.yaml - markdown linting rules

All checks use pinned action versions with SHA for security.
Workflow triggers on PR changes to charts/** paths.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <f@lex.la>

.yamllint.yaml

@@ -0,0 +1,17 @@
+# yamllint configuration
+# https://yamllint.readthedocs.io/
+
+extends: default
+
+rules:
+  line-length:
+    max: 120
+    level: warning
+  document-start: disable
+  truthy:
+    allowed-values: ['true', 'false', 'on', 'off']
+  comments:
+    min-spaces-from-content: 1
+  indentation:
+    spaces: 2
+    indent-sequences: true