docs: populate CONTRIBUTING.md with full contribution guide

[Copilot]

CONTRIBUTING.md was completely empty. This PR replaces it with a comprehensive guide tailored to the ExecuTrace project.

What's added

• Code of Conduct — links to CODE_OF_CONDUCT.md and the enforcement contact
• Ways to Contribute — quick-reference table covering bugs, features, docs, tests, and security
• Getting Started — fork/clone, virtual env setup, editable install (pip install -e .), and smoke-test
• Project Structure — annotated directory tree of the repo
• Running Tests — single-command instructions (python test_comprehensive.py)
• Coding Standards — PEP 8, type annotations (3.9+ syntax), get_logger usage, dependency policy
• Commit Conventions — conventional-commit-style prefixes with examples (feat, fix, docs, test, refactor, chore, ci)
• PR Process — step-by-step from branch creation to merge, with a note to open an issue before large changes
• Bug Reporting / Feature Requests — issue checklists with required fields
• Documentation — how to update the website and contributors.json
• Security — pointer to SECURITY.md responsible-disclosure process