readme: update

Rewrite the README and the source comments in a plainer register.

The README had picked up the usual generated-prose tics: seventeen em
dashes, inline bold headers over every work item, signposting lines like
'the two scripts are called out because it matters', and a two-word
'Twenty of them.' dropped in for emphasis. Replaced with commas, real
section headings, and sentences that carry their own subject.

Also fixes the two rustdoc warnings cargo doc was emitting: <NAME> and
<N> in the subcommand doc comments were being parsed as unclosed HTML
tags. cargo doc is clean now.

src/error.rs

@@ -1,7 +1,7 @@
 //! One error type for the whole CLI.
 //!
-//! Anything that can go wrong ends up as an [`Error`], gets a `?` at the call
-//! site, and is printed once in `main` — no `unwrap` in the command paths.
+//! Everything that can go wrong ends up as an [`Error`], gets a `?` at the
+//! call site, and is printed once by `main`.
 
 use std::path::PathBuf;