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

The old text served as the list of what to cover. 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; the pull request lists
each disagreement. Sentences stay under about 30 words, tables are lists,
and no issue links or project history remain in text for users. Two code
comments now name the new README sections, and the one in
`src/cli-log.ts` matches "What quak prints" on stderr, stdout and exit
status; a third no longer cites the README.

Model: fable-5-1

src/cli-log.ts

@@ -11,7 +11,8 @@
 // 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.
+// command line that quak cannot parse, and the usage printed when no command
+// is given.
 //
 // The library itself writes no lines: its layers report through callbacks, and
 // the CLI logs what they report. `logRequests` gives the API client options