fix(ci): use two-step yq approach for manifest merging (AndyMik90#1408)

The 'add' function doesn't exist in yq v4, causing parse errors.
Use a two-step approach that was tested locally:
1. Collect all files with '[.files] | flatten'
2. Load merged files into first manifest with 'load()'

Tested locally with yq v4.50.1

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

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fix(ci): fix YAML syntax error in release.yml multiline string

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