docs: add CONTRIBUTING.md with build instructions and PR workflow (Closes #3)

[18680368135]

Summary

Add comprehensive CONTRIBUTING.md with setup instructions, build commands, and PR workflow guidelines.

Changes

• Added CONTRIBUTING.md covering cloning, dependencies, build commands, diagnostic logs, PR workflow, code style, and testing
• Links to pull_request_template.md
• References .editorconfig formatting rules

Testing

• Verified file exists with all required sections
• Verified build commands documented (python3 build.py)
• Verified PR workflow steps (fork, branch, commit, pull request)
• Verified link to pull_request_template.md
• Verified .editorconfig reference
• Verified cloning and dependency instructions

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR
• Documentation has been updated, if applicable
• Configuration or schema changes are documented, if applicable
• No generated build artifacts are committed, except the required diagnostic build log
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered

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• I would like to request that my diagnostic build log is removed before merging