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

CONTRIBUTING.md

@@ -38,11 +38,13 @@ chore/PP-7-update-dependencies
 ### Useful Git Commands
 
 List all feature branches for this project:
+
 ```sh
 git branch -l 'feat/PP-*'
 ```
 
 List all branches for a specific issue:
+
 ```sh
 git branch -l '*/PP-10-*'
 ```
@@ -52,11 +54,13 @@ git branch -l '*/PP-10-*'
 ### Title Format
 
 Keep PR titles human-friendly and descriptive:
+
 ```
 Upgrade backend to Python 3.14 with exact dependency pinning
 ```
 
 Or if you prefer structured titles:
+
 ```
 [feat] PP-10: Upgrade backend to Python 3.14 with exact dependency pinning
 ```
@@ -64,17 +68,21 @@ Or if you prefer structured titles:
 ### PR Description
 
 Include the issue reference so Gitea can auto-link:
+
 ```markdown
 Fixes PP-10
 
 ## Summary
+
 Brief description of changes.
 
 ## Changes
+
 - Bullet 1
 - Bullet 2
 
 ## Testing
+
 How to verify this PR works.
 ```
 
@@ -92,6 +100,7 @@ Fixes PP-10
 ```
 
 Example:
+
 ```
 feat: Add startup compatibility validation
 
@@ -104,6 +113,7 @@ Fixes PP-10
 ## Code Quality Gates
 
 All commits must pass local pre-commit hooks:
+
 - `ruff` (format + lint)
 - `pyright` (type checking)
 - `pytest` (tests + coverage ≥95%)
@@ -111,6 +121,7 @@ All commits must pass local pre-commit hooks:
 - `xdoctest` (doctest extraction)
 
 Run locally before pushing:
+
 ```sh
 cd backend
 uv run pytest
@@ -121,6 +132,7 @@ uv run pytest
 See [README.md](README.md#manual-setup-if-not-using-docker) for manual setup.
 
 For Docker development:
+
 ```sh
 docker compose -f compose.dev.yml up
 ```