docs: improve Code Style section with actionable guidance

[findmail]

Summary

Fixes #4421

Problem

The Code Style section told contributors to "follow existing code conventions" without explaining:

• What those conventions are
• What formatter to use
• How to run formatting checks
• How to troubleshoot issues

This circular guidance was unhelpful for new contributors.

Solution

Added Explicit Tool Information

• ✅ Specified Ruff as the Python formatter
• ✅ Listed line length (100 chars) and Python version (3.11+)
• ✅ Clarified markdownlint configuration

Provided Actionable Commands

# Format all Python files
ruff format .

# Check for linting issues
ruff check .

# Auto-fix linting issues
ruff check --fix .

Added Pre-commit Guidance

• Installation instructions
• What gets checked
• How to run manually
• How to troubleshoot failures

Added Troubleshooting

• Common pre-commit failure scenarios
• Step-by-step fixes
• "Getting Help" section

Impact

New contributors now have:

• Clear, actionable commands instead of vague directives
• Specific tool names and configurations
• Troubleshooting guidance
• A path to get help

Testing

• ✅ Checked that Ruff configuration matches pyproject.toml
• ✅ Verified markdownlint rules reference exists
• ✅ Commands tested locally

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Fixes #4421