fix: improve shell detection, curl flags, and cleanup robustness

- Use ZSH_VERSION instead of ZSH_NAME for reliable zsh detection
- Document NVIM_* env vars as optional external overrides
- Add -fsSL flags to curl for mise bootstrap
- Consolidate duplicate shellcheck directives in install-fonts.sh
- Add explicit return 0 to cleanup script helper functions
- Add actionable remediation to uv-not-found error message

scripts/cleanup-old-version-managers.sh

@@ -37,6 +37,7 @@ remove_dir()
   else
     msgr ok "$label not found (already clean): $dir"
   fi
+  return 0
 }
 
 remove_file()
@@ -51,6 +52,7 @@ remove_file()
       msgr run_done "Removed $label: $file"
     fi
   fi
+  return 0
 }
 
 msgr msg "Cleaning up old version manager installations..."