"ERROR: stale docmap docs: entries" had a vestigial "docs:" fragment
that reads awkwardly (looks like a YAML reference).
Change to: "ERROR: stale docmap entries (paths do not exist):"
Addresses NIT finding in review #4175.
Non-git tools (e.g. `find`, `ls`) can emit paths with a "./" prefix.
Without stripping this, "./cmd/foo.go" would not match the glob "cmd/**",
producing a false-positive uncovered-file failure.
Fix: add strings.TrimPrefix(f, "./") after backslash normalization.
Test: TestRunValidateDocmap_DotSlashPrefix
Addresses MINOR finding in review #4175.
Add IsRegular() check after Lstat so directories, FIFOs, and device nodes
produce a clear error ("docmap must be a regular file") instead of a
confusing downstream parse error.
Test: TestValidateDocmapPath_NonRegularFile
Addresses MINOR finding in review #4175.