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)

TODO.md

@@ -29,6 +29,14 @@
 
 # Completed Steps
 
+- `README.md` rewritten from scratch, sentence by sentence, as a newcomer reads
+  it. It leads with what the program is and how to use it. A reference section
+  covers every setting and every subcommand. Development and testing come last.
+  Every statement was checked against the code on `next`. Five comments in
+  `main_test.go` cite "Settings", where their rules now live. One comment each
+  in `scan_test.go`, `trees_test.go` and `exclude_test.go` cites what the README
+  now says (2026-11-02, https://git.eeqj.de/sneak/sfdupes/issues/135)
+
 - each record holds `disk_size`, the bytes the file takes on disk: its allocated
   blocks, `st_blocks` × 512, from the lstat the walk already does. `trees`
   prints a `disk_size` column after `size`, the sum over the `dupe` copy's