Run npm verbosely when debug mode is enabled

[ekamran]

When an npm command hangs or misbehaves during preparation, the runner shows nothing about where it is stuck. The original hang report (#222) was resolved by a Node version bump in #225, but that fixed only that incident. A future npm hang still produces no output to diagnose.

This adds npm verbosity behind the existing debug mode, as suggested in the #221 discussion: when WPT_DEBUG is set, the npm install and npm run build commands in prepare.php run with --loglevel=silly, which is npm's maximum logging level and the long form of the -ddd flag from #221. Default behavior is unchanged. The gate mirrors how debug mode already adds verbosity to the rsync operations in the same file.

README.md and .env.default are updated in the places that document WPT_DEBUG, so the sample env file and the docs match the behavior.

This supersedes #221 by @rollecode, whose PR established the idea. It was based on an older prepare.php from before the runner_vars refactor, so this is a fresh commit on current master with the WPT_DEBUG gate the reviewers asked for.

Verification:

• With WPT_DEBUG unset, the generated command is byte identical to master.
• With WPT_DEBUG set, both npm commands get --loglevel=silly. Verified locally on npm 11 that npm consumes the flag itself, including after the script name in npm run build, and emits silly level output.
• php -l and phpcs pass on the touched files.

Use of AI

AI assistance: Yes
Tool(s): Claude Code and Codex
Used for: Investigation, implementation review, edge-case testing, and PR wording. I reviewed the reasoning and test results, and I take responsibility for the contribution.

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Co-authored-by: ekamran <ekamran@git.wordpress.org>
Co-authored-by: kittenkamala <amykamala@git.wordpress.org>
Co-authored-by: mindctrl <mindctrl@git.wordpress.org>

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[mindctrl]

This looks correct to me.

Nits:

• It might read better to move the comment and variable assignment before the comment block right above it, so the existing comment stays attached to the perform_operations() call it explains.
• The multiline comment style is drifting here and other parts of the file - // vs /**.