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/spec-template.md

@@ -113,4 +113,3 @@
 - **SC-002**: [Measurable metric, e.g., "System handles 1000 concurrent users without degradation"]
 - **SC-003**: [User satisfaction metric, e.g., "90% of users successfully complete primary task on first attempt"]
 - **SC-004**: [Business metric, e.g., "Reduce support tickets related to [X] by 50%"]
-