Read runtime version from a version file (.node-version, .tool-versions, etc.)

[laura-mlg]

Problem statement

Today, runtime versions in qlty.toml have to be specified as literal strings:

[runtimes.enabled]
node = "20.11.0"

Many repositories already have a canonical version file checked in for their toolchain — .node-version (nvm, Volta, fnm), .tool-versions (asdf, mise), .nvmrc, .python-version (pyenv), .ruby-version (rbenv, chruby), etc. Maintainers have to either:

1. Duplicate the version into qlty.toml, and remember to keep both in sync on every upgrade, or
2. Omit it and rely on Qlty's default version, which may drift from the version their app actually runs on.

This is a common source of subtle drift — Qlty runs against a different Node/Python/Ruby than the rest of the toolchain does.

Solution brainstorm (optional)

Let qlty.toml point at an existing version file instead of requiring an inline version. Something like:

[runtimes.enabled]
node-version-file = "client/.node-version"

Ideally support the common formats per runtime:

• Node: .node-version, .nvmrc
• Python: .python-version
• Ruby: .ruby-version
• Multi-runtime: .tool-versions (asdf/mise)

This would let teams keep a single source of truth for the runtime version their project uses.