feat: make buildernet vm data disk size configurable with env var

[fkondej]

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[claude]

Claude finished @fkondej's task in 1m 1s —— View job

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PR Review: Make BuilderNet VM data disk size configurable

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The change itself is clean and correct — replacing the hardcoded 100G with ${VM_DATA_DISK_SIZE:-100G} is the right pattern and the default preserves backward compatibility.

One issue: missing documentation. The new VM_DATA_DISK_SIZE env var needs to be added to:

1. The script header comment (lines 8-9 of prepare.sh)
2. The README's Environment Variables table

See inline comment for details.