DEBUG log level prepends "Quarto version: X.Y.Z" line to stderr for all commands

When QUARTO_LOG_LEVEL=DEBUG is set (or GitHub Actions debug mode auto-elevates the level), every Quarto subcommand prepends a bare Quarto version: X.Y.Z line to stderr — including commands whose stderr is the data channel that consumers parse, such as quarto list extensions.

Reproduction:

$ QUARTO_LOG_LEVEL=DEBUG quarto list extensions
Quarto version: 1.9.37
Id                        Version    Contributes
quarto-ext/fontawesome    1.3.0      shortcodes

The same line shows up on error output of other commands (quarto inspect, quarto render with a fatal error, etc.).

Source of the diagnostic:

https://github.com/quarto-dev/quarto-cli/blob/36e2232273cb9de9af82f2bbb946a0a36808b425/src/quarto.ts#L161

The log level is auto-elevated to DEBUG when GitHub Actions runs with debug logging on (RUNNER_DEBUG=1):

https://github.com/quarto-dev/quarto-cli/blob/36e2232273cb9de9af82f2bbb946a0a36808b425/src/core/log.ts#L104-L111

Impact: any tool that parses Quarto CLI stderr breaks under DEBUG. The R quarto package's quarto_list_extensions() parses quarto list extensions stderr with read.table(header = TRUE); under DEBUG, the prepended version line is consumed as the header row and downstream code fails with argument 1 is not a vector. We hit this in quarto-dev/quarto-r CI any time the workflow is re-run with debug logging.

We could:

• Route this diagnostic through the structured log handler (with level prefix and timestamp) so it lands in --log output rather than as a bare line on stderr, or
• Gate it on TRACE rather than DEBUG, since DEBUG is auto-enabled by CI debug mode and a number of code paths legitimately want DEBUG without the version preamble polluting stdout/stderr consumers.

Either approach keeps the diagnostic available without breaking programmatic consumers of CLI output.