fix(bump): auto-increment prerelease when tag already exists

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• When cutting a prerelease with --prerelease, we now check existing git tags (respecting tagPrefix) for the same base version and prerelease channel.
• If a tag already exists (e.g., v1.4.3-xyz.0), we auto-increment to the next available (e.g., v1.4.3-xyz.1) before tagging.
• Also supports unnamed prereleases (e.g., -0, -1), applying the same collision-avoidance.
• Adds integration tests for both named and unnamed prereleases in package.json and gitTagFallback flows.

Fixes: #211