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

.pre-commit-config.yaml

@@ -13,6 +13,14 @@ repos:
       - id: check-toml
       - id: mixed-line-ending
 
+  # Markdown linting
+  - repo: https://github.com/igorshubovych/markdownlint-cli
+    rev: v0.45.0
+    hooks:
+      - id: markdownlint
+        args: [--fix]
+        files: \.(md|markdown)$
+
   # TOML linting
   - repo: https://github.com/macisamuele/language-formatters-pre-commit-hooks
     rev: v2.14.0
@@ -86,10 +94,10 @@ repos:
       # Prettier with auto-format for pre-commit (CI uses --check)
       - id: prettier
         name: prettier
-        entry: bash -c 'export PATH="$HOME/.local/share/mise/shims:$HOME/.local/bin:$PATH" && cd frontend && if [ "${CI:-}" = "true" ]; then corepack yarn prettier --check src/; else corepack yarn prettier --write src/; fi'
+        entry: bash -c 'export PATH="$HOME/.local/share/mise/shims:$HOME/.local/bin:$PATH" && FILES=(); for file in "$@"; do FILES+=("$(realpath "$file")"); done && if [ "${CI:-}" = "true" ]; then corepack yarn --cwd frontend prettier --check "${FILES[@]}"; else corepack yarn --cwd frontend prettier --write "${FILES[@]}"; fi' --
         language: system
-        files: ^frontend/.*\.(js|ts|vue|json|css|scss|md)$
-        pass_filenames: false
+        files: \.(js|ts|vue|json|css|scss|md|markdown)$
+        pass_filenames: true
 
       # TypeScript type checking (same as CI)
       - id: typescript-check