README.md rewritten sentence by sentence (closes #219)

The old text served as the list of what a reader needs covered. The new
text leads with what quak is, what it is for and how to use it. Every
command, option, exit code and file layout follows in reference sections,
then the library reference, then design, entrypoints and development.
Every statement was checked against the code on next; where the old text
disagreed, the pull request lists it. No sentence runs past about 30
words, every table is now a list, and no issue links or project history
remain in text meant for users. Two code comments that named README
sections now name the new ones; a third no longer cites the README.

Model: fable-5-1

src/cli-log.ts

@@ -1,17 +1,15 @@
 // The CLI's log level, and the one function the commands write stderr lines
 // through.
 //
-// Every line quak logs goes to stderr, at one of six levels, from the
-// quietest: fatal, error, warning, info, debug, trace. A run prints the lines
-// at its own level and at every quieter one. Its level is info unless `-v` or
-// `-q` asks for another (see "What quak prints" in the README). A command that
-// runs and exits non-zero logs a fatal line saying why, so even `-q`, which
-// prints only fatal lines, never leaves such an exit unexplained. An error
+// Every line quak prints on stderr is a log line at one of six levels, from
+// the quietest: fatal, error, warning, info, debug, trace. A run prints the
+// lines at its own level and at every quieter one. Its level is info unless
+// `-v` or `-q` asks for another (see "What quak prints" in the README). Every
+// command that exits non-zero logs a fatal line saying why, so even `-q`, which
+// prints only fatal lines, never leaves a non-zero exit unexplained. An error
 // line reports something that failed, such as a file or an album, and a
 // warning something that did not fail but needs the user's attention. stdout
-// carries a command's output and the prompts of `login`, and is not a log.
-// Two things on stderr are not log lines: the `error:` message for a
-// command line quak cannot parse, and the usage `quak` alone prints.
+// carries only a command's output and is not a log.
 //
 // The library itself writes no lines: its layers report through callbacks, and
 // the CLI logs what they report. `logRequests` gives the API client options