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>

.markdownlint.yaml

@@ -0,0 +1,16 @@
+# markdownlint configuration
+# https://github.com/DavidAnson/markdownlint
+
+default: true
+
+# MD013/line-length - Line length
+MD013:
+  line_length: 120
+  tables: false
+  code_blocks: false
+
+# MD033/no-inline-html
+MD033: false
+
+# MD041/first-line-heading
+MD041: false