fix: add markdownlint config and auto-fix markdown errors

- Add .markdownlint-cli2.jsonc based on headlamp-agent-skills config
- Disable MD013 (line length), MD041 (first-line-heading), MD036
  (emphasis-as-heading), MD024 (no-duplicate-heading), MD040
  (fenced-code-language), MD060 (table-column-style), MD029
  (ol-prefix), MD033 (no-inline-html) — appropriate for plugin docs
- Run markdownlint-cli2 --fix to auto-fix MD022, MD031, MD032
- Manually fix remaining code block language annotations

Co-Authored-By: Paperclip <noreply@paperclip.ing>

README.md

@@ -206,7 +206,7 @@ For complete testing guide including CI/CD integration, see **[docs/TESTING.md](
 
 ## Project Structure
 
-```
+```text
 src/
   index.tsx                           -- Entry point. Registers sidebar entries, routes, and error boundaries.
   test-utils.tsx                      -- Shared test fixtures (makeResult, makeAuditData).
@@ -236,7 +236,7 @@ GET /api/v1/namespaces/polaris/services/polaris-dashboard/proxy/results.json
 
 This endpoint is served by the `polaris-dashboard` ClusterIP service, which is created by the Polaris Helm chart when `dashboard.enabled: true`. The JSON response matches Polaris's `AuditData` schema (`pkg/validator/output.go`):
 
-```
+```text
 AuditData
   ClusterInfo      -- nodes, pods, namespaces, controllers
   Results[]        -- per-workload results