Rewrite README.md sentence by sentence for a newcomer

The old README was a specification written as one dense document:
sentences past 30 words, lists chained with commas, exceptions ahead
of the rules, and release history in the user text. This rewrite
leads with what the program is, what it is for and how to use it,
then gives a reference for the settings, each subcommand, the
database, the output formats, the progress display and the exit
codes. Non-goals come before the reference; the constraints, the
entrypoints and the build come last. Every statement was checked
against the code on next and against what the built binary prints on
a small tree. The section names the code comments cite are kept, and
three comments in main_test.go now cite Settings. TODO.md records the
step.

Model: fable-5-1

TODO.md

@@ -31,10 +31,10 @@
 
 - `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.
+  covers the settings, the database, the output formats and the exit codes;
-  Every statement was checked against the code on `next`. Three comments in
+  development and testing come last. Every statement was checked against the
-  `main_test.go` cite "Settings", where their rules now live (2026-10-26,
+  code on `next`. Three comments in `main_test.go` cite "Settings", where their
-  https://git.eeqj.de/sneak/sfdupes/issues/135)
+  rules now live (2026-10-26, https://git.eeqj.de/sneak/sfdupes/issues/135)
 
 - a file or directory removed while the scan runs is not an error, in the walk
   or in the hash phase. The scan prints no line for it and does not count it as