scratch: the scratchpad is the inbox, with ways out and a search

A note now leaves the scratchpad by becoming something else: `K` keeps it
as a document, as before; `C` makes it a card where `:card` would put one,
the first line its title; `A` opens `:append` with the document on screen
filled in, Tab completing any other title, and puts the note at its end.

`/api/capture` takes `to: "note"` and `to: "document"` (named as a link
names it), and `note`, the scratch note the text came from, dropped once
the text has landed. Appending keeps the revision before it and indexes
links and media, as a save does.

Migration 0028 adds `scratch_fts`; `GET /api/scratch?q=` searches it, and
`/` lists note hits under "notes" after the cards, Enter lowering the
drawer at the note.

The `c` capture line cycles its target with Tab: a card, a note, or a
timed line in today's entry, which `:today <text>` now writes.

docs/wiki/Guide.md

@@ -84,7 +84,10 @@ More in [Writing](Writing.md).
 
 In a meeting someone asks you to renew a certificate. Press `c`, type
 `Renew the gateway certificate`, and `Enter`. The card is in this space's
-inbox, and you have not left the document you were reading.
+inbox, and you have not left the document you were reading. Not a task? `Tab`
+on the same line makes it a note on the scratchpad, and `Tab` again a timed line
+in today's diary entry. A note waits there until `K`, `C` or `A` turns it into a
+document, a card, or more of a document.
 
 Press `b` to see the **board**, in a tile beside the document. A digit there
 shows the board of another space.