boards: a card can come back once it is done

`:repeat weekly` gives a card a rule: daily, weekly, monthly, yearly, or
every +Nd, +Nw, +Nm, +Ny. Finishing it, by a move into the last list or by
the API's done, leaves a copy at the bottom of the inbox with the same
title, notes, document and rule, due on the next date. The finished card
gives its rule up in the same transaction, so reopening it and finishing it
again makes no second copy.

The next date is counted on the calendar in WORKSPACE_TIMEZONE, so noon
stays noon across a change of the clocks, and months are counted from the
old due date rather than step by step, so a card due on the 31st comes back
on the 31st wherever a month has one. A card finished late skips the dates
already past; one without a date counts from the moment it was finished.

A move and an edit answer with `again`, the copy, so the status line can
say when the card comes back. The board and the agenda show the rule as
`↻ weekly`, the rail has a button for it, and `:` in the agenda now opens
the line with the agenda kept up, so `:repeat` knows which card.

The rule travels in `.workspace.json` without a format bump: an older build
ignores the field and imports the card as one that does not repeat. An
import drops a rule it cannot read.

README.md

@@ -137,7 +137,7 @@ into the folder being browsed, and so does dropping them on the list. `:name <te
 the cursor — a document, a folder, or a card on a board — and `:name!` renames
 the space itself. The board has
 `:board`, `:card <title>` (`:card!` links the open document to it),
-`:due <when|->`, `:done` and `:list <name>` (`:list!` deletes one).
+`:due <when|->`, `:repeat <rule|->`, `:done` and `:list <name>` (`:list!` deletes one).
 
 On a touch device modal editing is off by default (`:set vim` turns it on) and
 the same actions are buttons: the `⋯` on a row — or a long press — opens what
@@ -233,13 +233,30 @@ deleting the document leaves the card standing, because the work is still open
 once its notes are gone. Only a document can be a card's notes; a folder or a
 file is refused.
 
+### Cards that come back
+
+`:repeat weekly` (or the rail's ↻ button) makes a card come back once it is
+done. The rules are `daily`, `weekly`, `monthly`, `yearly`, and every `+3d`,
+`+2w`, `+6m` or `+1y`; `:repeat -` stops it. Finishing the card, on the board
+or in the agenda, leaves a copy at the bottom of the inbox with the same title,
+notes, document and rule, due on the next date, and the status line says when.
+The next date counts on from the old due date, in `WORKSPACE_TIMEZONE`, so a
+card due at noon stays due at noon across a change of the clocks. A card due on
+the 31st comes back on the last day of a shorter month and on the 31st again
+where there is one. A card finished late skips the dates that have passed and
+comes back on the first one from today on; a card without a due date counts from
+the moment it was finished. The finished card gives up its rule, so reopening it
+and finishing it again makes no second copy. The board and the agenda show the
+rule as `↻ weekly` beside the date.
+
 ## The agenda
 
 `t` (or `:agenda`) gathers every unfinished card from every board that is late,
 due within the week, or sitting in a column between the inbox and done — the
 work already in hand — and says whether the diary has an entry for today. `j`/`k`
 walk it, `Enter` goes to the card on its own board, `Space` finishes it where
-it stands, `D` sets its due date (`:due` on the line), `H` / `L` send it to
+it stands, `D` sets its due date (`:due` on the line), `:` opens the line with
+the agenda kept up so `:repeat` acts on the card, `H` / `L` send it to
 the list left / right on its board, and `T` writes today's entry. The bar shows `late` and `today` counts
 whenever either is above zero, and the agenda opens by itself once a day when
 something is late or due, so nothing has to be remembered to be looked at.
@@ -362,7 +379,9 @@ other embedded file in `media/`, under their real names, and relative links
 between documents, folders and files, so the archive reads in any markdown
 viewer without this application. What plain files have no place for — when
 each entry was written, names as they were typed, which space is the diary,
-every board with its cards — goes into one `.workspace.json` beside them.
+every board with its cards and their repeat rules — goes into one
+`.workspace.json` beside them. An older build reads a newer file's cards
+without their rules rather than refusing it.
 
 The diary is the diary by role, not by name: rename it and the reminder and
 the agenda still know which space it is, and it cannot be deleted — empty it
@@ -780,8 +799,8 @@ sealed or not.
 | `POST` | `/api/spaces/{id}/lists` | add a list |
 | `PUT`/`DELETE` | `/api/lists/{id}` | rename / delete a list and its cards |
 | `POST` | `/api/lists/{id}/cards` | add a card |
-| `PUT`/`DELETE` | `/api/cards/{id}` | update (title, body, due, done, link) / delete |
-| `POST` | `/api/cards/{id}/move` | to another list and place in it |
+| `PUT`/`DELETE` | `/api/cards/{id}` | update (title, body, due, repeat, done, link) / delete; finishing a repeating card answers its copy as `again` |
+| `POST` | `/api/cards/{id}/move` | to another list and place in it; `again` as above |
 | `POST` | `/api/nodes/{id}/move` | reparent, and place among the new siblings |
 | `POST` | `/api/nodes/move` | `{ ids, parent_id }`: several into one folder, all or none |
 | `POST` | `/api/nodes/{id}/import` | import a folder or zip (multipart) under this node |