fix(config): fallback when user info unavailable

[nexxeln]

Closes #29292

Summary

• fall back to "user" when os.userInfo() is unavailable during config startup
• apply the same inline fallback to macOS managed preference lookup
• add a regression test for missing system user info

Validation

• bun test test/config/config.test.ts -t "falls back to generic username"
• bun turbo typecheck via pre-push

[YOMXXX]

Thanks for landing the smaller version.

One note: #29296 identified the same os.userInfo() fallback issue and also covered the managed preferences call site, which this PR adopted. Could you add a short note linking #29296 as prior work / inspiration? That would make the contribution trail clear.