On Windows npm prints only a terse summary on failure; the actual cause
(postinstall stderr like Electron's install.js, network traces, EBUSY
retries) lives in npm-cache\_logs\<ts>-debug-0.log, which never reached
the Tauri bootstrap log. Field report: a fresh-VM desktop install died
with 'npm error command node install.js' and zero actionable detail.
Adds Write-NpmDebugLogTail: locates the debug log from npm's 'A complete
log of this run' line (fallback: newest _logs/*-debug-*.log under 'npm
config get cache') and replays its last 200 lines through our output
stream, which the bootstrap installer's streaming sink captures.
Wired at all four npm failure sites: desktop workspace npm ci/install,
_Run-NpmInstall (browser tools), Install-AgentBrowser (--silent global
install), and the desktop 'npm run pack' build step.
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