chore(copilot): remove duplicate agent files with .md extension

Remove duplicate agent files that use `.md` extension. The `.agent.md`
extension is the correct convention for agent definition files as it:

1. Provides semantic clarity (agent definitions vs regular docs)
2. Allows mdformat exclusion to preserve YAML frontmatter
3. Distinguishes from regular markdown documentation

Files removed:
- ci-cd-specialist.md
- code-reviewer.md
- documentation-writer.md
- project-coordinator.md
- python-developer.md
- task-planner.md
- tdd-specialist.md

All agents now use only `.agent.md` extension.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Doug Borg