fix: report the shim auto-install step that failed

[coreybutler]

Summary

• Shim auto-install no longer collapses every failure into AutoInstallFailed.
• A missing nvm.exe, a failed start, a non-zero nvm install, a prompt read failure, and a version that still does not resolve each print NVM4307 through NVM4311.
• Pins shim 26c53f6.

Test plan

• zig build test -Dapp=node and zig build test -Dapp=proxy
• Messages include the event code and do not contain AutoInstallFailed
• The command runner distinguishes a missing executable, a failed spawn, exit code 9, and exit code 0
• Auto-install stays off when disabled or when a version is passed explicitly

Fixes #1445