feat(cli): prompt to run npm init when package.json is missing

[jheddings]

Summary

• we init now detects a missing package.json and asks the user if they'd like to run npm init instead of failing immediately
• If the user accepts, npm init runs interactively and we init continues scaffolding once it completes
• If the user declines, the command exits cleanly with a short redirect message
• Adds --yes/-y flag that bypasses the confirmation prompt and passes -y through to npm init for non-interactive use
• In non-TTY contexts (CI, piped stdin), the prompt declines automatically rather than blocking on a closed stream — pass --yes to opt in
• Drops shell: true from the npm install/npm init spawn calls to silence Node's DEP0190 deprecation; uses npm.cmd on Windows where the .cmd shim is required

Closes NW-18265
Replaces #71 (closed when branch was renamed)

Test Plan

• we init in a directory with no package.json — verify prompt appears
• Accept at the prompt — verify npm init runs interactively, then scaffolding continues
• Decline at the prompt — verify clean exit with message (no stack trace)
• we init -y in a directory with no package.json — verify no prompt and npm init -y is used
• we init with stdin piped from /dev/null — verify clean exit with redirect message (no hang)
• we init in a directory that already has package.json — verify existing behavior unchanged
• All unit tests passing (just test)

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