feature/pp-58-runtime-image-contract (#68)

## Summary

This PR tightens repository quality enforcement around markdown and documentation. It adds `markdownlint` to the `cicd-checks` workflow, expands pre-commit coverage so markdown files are checked repo-wide, and cleans up the PP-58 documentation set to keep it aligned with the new policy.

## What changed

- Added a `Markdownlint Check` entry to `.gitea/workflows/cicd-checks.yaml`
- Added `markdownlint` to pre-commit and widened prettier coverage to include markdown files across the repo
- Updated `README.md` to satisfy markdownlint line-length rules
- Normalized the PP-58 documentation set:
  - `docs/DEPLOYABLE_RUNTIME_CONTRACT.md`
  - `docs/adr/ADR003-deployable_runtime_image_contract.md`
  - `docs/DEVELOPMENT.md`
  - `docs/CICD_MULTI_STAGE_BUILD.md`
  - `docs/CICD_TROUBLESHOOTING_GUIDE.md`
  - `docs/SECURE_DOCKER_CICD.md`

## Validation

- `pre-commit run markdownlint --files README.md docs/DEPLOYABLE_RUNTIME_CONTRACT.md`
- `pre-commit run prettier --files README.md docs/DEPLOYABLE_RUNTIME_CONTRACT.md`
- Workflow YAML validation returned no errors

## Notes

This change does not alter application runtime behavior. It only strengthens CI and documentation quality enforcement.

Co-authored-by: copilotcoder <copilotcoder@darkhelm.org>
Reviewed-on: #68

frontend/tests/unit/App.test.ts

@@ -1,10 +1,10 @@
-import { describe, it, expect } from 'vitest'
-import { mount } from '@vue/test-utils'
-import App from '@/App.vue'
+import { describe, it, expect } from 'vitest';
+import { mount } from '@vue/test-utils';
+import App from '@/App.vue';
 
 describe('App.vue', () => {
   it('renders properly', () => {
-    const wrapper = mount(App)
-    expect(wrapper.text()).toContain('Plex Playlist')
-  })
-})
+    const wrapper = mount(App);
+    expect(wrapper.text()).toContain('Plex Playlist');
+  });
+});