docs: add CONTRIBUTING.md

[Cool-fire]

Add a new root-level CONTRIBUTING.md with the requested managed header and contribution guidance covering code style, Conventional Commits, and pull request expectations.

Verification:

• Verified CONTRIBUTING.md content matches the requested text exactly.

Limitations:

• No tests or build steps were needed for this documentation-only change.

Files changed

CONTRIBUTING.md | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

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