fix(migrate): keep existing .nvmrc pins

[liangmiQwQ]

vp migrate currently converts .nvmrc to .node-version and deletes .nvmrc. This was added in #1159, before Vite+ could read .nvmrc. Since #2244, Vite+ reads .nvmrc directly, and #2676 keeps .nvmrc pins in vp env pin.

The conversion now causes problems:

• Deleting .nvmrc breaks nvm, fnm, and other tools that still read it. Only actions/setup-node references were rewritten.
• It can change the resolved Node.js version. .nvmrc ranks below engines.node and devEngines.runtime, while .node-version ranks above them.
• The parser only read the first line, so .nvmrc files with comments were rejected.

This PR keeps .nvmrc as-is. Migration now only converts Volta's volta.node, and skips it when .nvmrc exists, because a new .node-version would take priority over that .nvmrc.

This PR also removes parseNvmrcVersion and the GitHub Actions node-version-file rewrite, which only existed for the removed file.

🤖 Generated with Claude Code

[liangmiQwQ]

@codex review

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-19T15:14:34.151556Z	98be74f	Manual request

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