Add CONTRIBUTING guide

[lxx197818]

Summary

Adds a root CONTRIBUTING.md for the project.

Fixes #3.

Changes

• Documented local setup expectations for the repository's polyglot modules.
• Added module build commands for backend, frontend, market, and Python tooling.
• Documented the required python3 build.py diagnostic workflow.
• Added the fork/branch/PR contribution workflow and linked .github/pull_request_template.md.
• Noted current formatting conventions because no root .editorconfig exists yet.

Testing

• git diff --check — passed.
• python -m compileall build.py tools — passed.
• python build.py — failed before building because Windows GBK output could not encode the warning symbol.
• PYTHONUTF8=1 python build.py — started but produced no output for over 30 seconds and did not create a fresh diagnostic artifact in this environment. I did not reuse the repository's stub diagnostic/build-00000000.* as a fresh run.

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