commands: one grammar for !, merged commands, a settings sheet, two-level help

A `!` now means force or everything and nothing else (`:w!`, `:search!`,
`:export!`, `:logout!`). A command that did a second thing behind its bang
takes a word for it instead: `:space new|name|rm`, `:list name|rm`,
`:task [add]|edit`, `:upload seal`, `:import zip [/]`. `:card` while reading
a document makes a card about it, so `:card!` is gone.

Merged: `:export` downloads what has focus, a file as it is or a document as
markdown (`:download` folded in); `:share [off|new]` replaces `:unshare` and
`:link`, and the list's `s` and `y` give way to it and the info sheet;
`:noh` replaces `:clear`; `:vault [unlock|lock|passwd]` replaces `:unlock`,
`:lock` and `:passwd`; Esc in the list puts down what `m` picked up before it
closes the document, replacing `:unmark`.

The old spellings run as their new lines for one release through a table in
commands.ts; help and Tab leave them out, and the wiki lists them.

A settings sheet (`,` or `:settings`) holds the options, the vault, the
backup, the signed-in browsers, ssh keys and capture tokens. Each row runs
the command a line would. Bare `:set`, `:vault`, `:backup`, `:sessions`,
`:sshkey` and `:token` open it at their part.

`?` shows the keys and commands to start with, flagged in the keymap and the
command table; a second `?` (or `:help all`) shows all of them, and a filter
always searches all of them. Help rows are keyed by key and description, as
two contexts can share a label.

docs/wiki/Guide.md

@@ -42,7 +42,7 @@ counts while either is above zero. More in [Boards and the agenda](Boards-and-Ag
 
 Press `2` for the `work` space. A **space** is one area of your life, with its
 own folders, its own board and its own search. Make more with
-`:space! <name>`; `w` shows all of them at once, with what was last written in
+`:space new <name>`; `w` shows all of them at once, with what was last written in
 each.
 
 ![Every space at a glance](../images/overview.png)
@@ -124,12 +124,12 @@ LaTeX between dollar signs and drawn by KaTeX:
 The slides came as a PDF. Drop it onto the list, or `:upload`, and it is a
 file in the folder, beside the notes about it. `Enter` opens a PDF, an image, a
 video, a CSV table, a text or source file, or a certificate in place. Anything
-else downloads with `:dl`.
+else downloads with `:export`.
 
 ![A CSV file as a table](../images/file-table.png)
 
 A scan of something private, like an ID or a contract, goes in with
-`:upload!` instead. That seals it in the **vault**, under a password that is
+`:upload seal` instead. That seals it in the **vault**, under a password that is
 never stored. See [The vault](Vault.md).
 
 ## On the way home: capture