feat(ci): add markdownlint-cli2 for consistent markdown formatting

Add automated markdown linting to ensure consistent formatting across
all markdown files in the repository.

Changes:
- Add .markdownlint-cli2.jsonc configuration
- Create .github/workflows/lint.yml for CI/CD integration
- Fix all 908 existing markdown errors across 27 files
- Enforce ATX-style headings and asterisk emphasis
- Set consistent 2-space list indentation

This prevents markdown errors after project initialization and
maintains high documentation quality standards.

templates/agent-file-template.md

@@ -3,20 +3,25 @@
 Auto-generated from all feature plans. Last updated: [DATE]
 
 ## Active Technologies
+
 [EXTRACTED FROM ALL PLAN.MD FILES]
 
 ## Project Structure
-```
+
+```text
 [ACTUAL STRUCTURE FROM PLANS]
 ```
 
 ## Commands
+
 [ONLY COMMANDS FOR ACTIVE TECHNOLOGIES]
 
 ## Code Style
+
 [LANGUAGE-SPECIFIC, ONLY FOR LANGUAGES IN USE]
 
 ## Recent Changes
+
 [LAST 3 FEATURES AND WHAT THEY ADDED]
 
 <!-- MANUAL ADDITIONS START -->