fix: handle pre-release versions in compareVersions

[notgitika]

Description

This commit fixes the update command implementation on preview releases.

The old implementation split on . and mapped all parts to Number, producing NaN for segments like "0-preview", and only compared the first 3 segments.

This caused all pre-release versions to appear equal, so agentcore update would report "already on the latest version" even when a newer pre-release was available.

The new implementation splits core version from pre-release at the first -, compares major.minor.patch numerically, then compares pre-release segments per semver precedence rules.

Related Issue

Closes #356

Documentation PR

N/A

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Other (please describe):

Testing

How have you tested the change?

• I ran npm run test:unit and npm run test:integ
• I ran npm run typecheck
• I ran npm run lint
• If I modified src/assets/, I ran npm run test:update-snapshots and committed the updated snapshots

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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