Rewrite README.md sentence by sentence for a newcomer

Rewrites `README.md` from scratch for #135, so that a newcomer can follow each sentence on one reading. It leads with what the program is and how to use it, then the reference, then development. Where the old text and the code disagreed, it states what the code does; #140 lists each point. Test comments that cited moved text cite the new headings.

Review scope: full reviews through the 16th, then only text changed since.
Dropped: the permission to add `github.com/spf13/viper`.

Model: fable-5-1 (rewrite); opus-5-5 (review, landing message)

exclude_test.go

@@ -17,7 +17,9 @@ import (
 func TestDefaultExcludes(t *testing.T) {
 	t.Parallel()
 
-	// README "Rules for the walk": each system's list.
+	// README "Rules for the walk": the Linux and macOS lists, and none
+	// on any other system. The Windows list exists in the code only: the
+	// binary does not build for Windows, so the README leaves it out.
 	cases := map[string][]string{
 		goosLinux: {"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp"},
 		goosDarwin: {