docs: add versioning guidelines and troubleshooting for tag push errors

[Copilot]

GitHub repository rules reject tags with versions older than existing tags (e.g., pushing v0.0.1 fails when v0.0.2 exists). This manifests as "push declined due to repository rule violations" and is a common source of confusion.

Changes

• CONTRIBUTING.md: Added semantic versioning workflow, tag creation guidelines, and explanation of GitHub's version ordering enforcement
• README.md: Added troubleshooting section with error diagnosis and resolution steps

Example

# Check existing tags first
git tag -l
# Output: v0.0.2

# Create next semantic version (not an older one)
git tag -a "v0.0.3" -m "Release v0.0.3"
git push origin v0.0.3  # ✓ succeeds

# This would fail:
git tag -a "v0.0.1" -m "Release v0.0.1"
git push origin v0.0.1  # ✗ push declined due to repository rule violations

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