Fix labeler.yml format for actions/labeler@v5

[Copilot]

actions/labeler@v5 changed the config schema — label entries must be arrays of config objects rather than plain glob strings, causing the workflow to fail with found unexpected type for label 'first-contribution' (should be array of config options).

Changes

• .github/labeler.yml: Migrate from v4 flat-glob format to v5 changed-files object format

# Before (v4)
first-contribution:
  - '**/*'

# After (v5)
first-contribution:
  - changed-files:
    - any-glob-to-any-file: '**/*'

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