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)

scan_test.go

@@ -1082,8 +1082,9 @@ func TestScanOneFilesystemWalksNestedFileOperand(t *testing.T) {
 	}
 }
 
-// buildSmokeTree recreates the README smoke-test filesystem layout
-// with deterministic content and returns the tree root.
+// buildSmokeTree recreates the tree of the test under README
+// "Definition of done" with deterministic content and returns the tree
+// root.
 func buildSmokeTree(t *testing.T) string {
 	t.Helper()