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

@@ -1,15 +1,17 @@
 // The CLI's log level, and the one function the commands write stderr lines
 // through.
 //
-// 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 "CLI surface" 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
+// 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
 // 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 only a command's output and is not a log.
+// 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.
 //
 // The library itself writes no lines: its layers report through callbacks, and
 // the CLI logs what they report. `logRequests` gives the API client options

test/cli/backup-end-to-end.test.ts

@@ -418,7 +418,7 @@ describe("the first backup of the account", () => {
         expect(downloads.sort()).toEqual([100, 200, 300]);
     });
 
-    // The tree "Backup layout" in the README describes. Each original is at
+    // The tree under "The backup directory" in the README. Each original is at
     // its save path, `YYYY/YYYY-MM/YYYY-MM-DD/YYYY-MM-DD.<fileID>.<ext>`, with
     // the extension of its title; the live photo is its image and its video,
     // each with the extension it had inside the ZIP, and a `.livephoto.json`

test/cli/log.test.ts

@@ -328,7 +328,7 @@ describe("what the library reports", () => {
         ]);
     });
 
-    // Each operation is named in plain words, as the README names it.
+    // Each operation is named in plain words.
     it("logs a refresh's progress at info, and its start and end at debug", () => {
         const { log, lines } = collect();
         const onProgress = logRefresh(log);