docs: a guide, a wiki and screenshots

The README grew a section per feature and is a good reference, but nothing
said where to begin. docs/wiki now has a getting-started page, a guide that
walks through one day with the workspace and explains each part where it comes
up, and a page per part of the app, each with screenshots of what it
describes.

Three pages are read out of the code rather than written, so they cannot drift
from it: the keys and commands from Help.svelte and commands.ts, the same lists
`:help` draws; the settings from .env.example, which gains a heading for the
vault, trash and history settings so they no longer sit under mail; and the
API from the README's table. `make wiki` writes them, and `--check` fails when
one is stale or a link between pages leads nowhere. `--publish` lays the pages
out for a GitHub wiki: links without `.md`, images beside them, `[[` escaped so
the wiki does not take a command's help text for a link to a page.

The screenshots are taken by `make screenshots`: the debug binary on a
temporary data directory, with a password made up for the run, filled with an
example workspace over the API, and a headless Chromium driven over the
DevTools protocol through the views the docs show. Nothing outside the
temporary directory is touched but docs/images, and the pictures are cut to
256 colours, a third of their size, which a dark UI does not show.

The README opens with the banner, a CI badge, the guide, the features grouped
by what they are for, and six of the screenshots.

docs/wiki/Boards-and-Agenda.md

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+# Boards and the agenda
+
+## The board
+
+Every space except the diary has a board. `b`, the board button on the rail, or
+`:board` swaps the tree for it, and `q` swaps back.
+
+![The board of a space](../images/board.png)
+
+A new board has three lists: `backlog`, `doing` and `done`. The lists mean
+something by where they stand, not by their names:
+
+- the **first** list is the inbox, where new cards land;
+- the **last** list is done. A card is finished exactly when it is in the last
+  list: moving it there stamps it, moving it out reopens it;
+- the lists between are **work in hand**, which the agenda shows.
+
+`:list <name>` adds a list in front of the last one, `:name!` renames the list
+under the cursor, and `:list!` deletes it with its cards. Done cards older than
+a week fold away until `z` shows them.
+
+| key | what it does |
+| --- | --- |
+| `h` / `l` | to the list on the left / right |
+| `j` / `k` | down / up the cards |
+| `J` / `K` | move the card down / up its list |
+| `H` / `L` | move the card to the list on the left / right |
+| `Space` | into the done list, or back out to the inbox |
+| `o`, `c` | a new card on this list |
+| `Enter` | the card's notes |
+| `D` | due date |
+| `R` | rename the card |
+| `z` | show done cards older than a week |
+| `F` | a letter over every card; type it to jump there |
+| `x`, `dd` | delete the card |
+| `q`, `Esc` | back to the tree |
+
+### Capturing a card
+
+`c` anywhere outside the board opens `:card ` on the command line. The card goes
+into the inbox of the space you are in, or of the first space with a board, and
+you stay where you were. From outside the browser, see [Capture](Capture.md).
+
+### Notes
+
+`Enter` on a card opens its notes: a document named after the card, made in the
+root of the space the first time. Closing it goes back to the board, on the
+same card. `:card!` while reading a document makes a card that points at it.
+
+The link is loose on purpose: deleting the document leaves the card, because the
+work is still open when its notes are gone.
+
+### Due dates
+
+`D` on a card (or `:due <when>`) takes a date the way it is said:
+
+| you type | it means |
+| --- | --- |
+| `today`, `td` | today |
+| `tm`, `tomorrow` | tomorrow |
+| `fri` | the next Friday, never today |
+| `+3d`, `+2w` | in three days, in two weeks |
+| `10-31` | the next 31 October |
+| `2026-10-31` | that day |
+| `-` | no due date |
+
+### Cards that come back
+
+`:repeat weekly`, or the ↻ button on the rail, makes a card come back once it
+is done. The rules are `daily`, `weekly`, `monthly`, `yearly`, and any `+3d`,
+`+2w`, `+6m` or `+1y`. `:repeat -` stops it.
+
+Finishing such a card leaves a copy at the bottom of the inbox, with the same
+title, notes and rule, due on the next date. The next date counts on from the
+old due date, in `WORKSPACE_TIMEZONE`:
+
+- 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;
+- 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
+does not make a second copy. The board and the agenda show the rule as
+`↻ weekly` beside the date.
+
+## The agenda
+
+`t` (or `:agenda`) gathers what needs you from every board at once.
+
+![The agenda](../images/agenda.png)
+
+- **late**, **today** and **this week**: unfinished cards by due date;
+- **in hand**: cards in a list between the inbox and done;
+- whether the diary has an entry for today;
+- **on this day**: diary entries from the same day in earlier years. A
+  29 February is remembered on the 28th in the years that have none.
+
+| key | what it does |
+| --- | --- |
+| `j` / `k` | down / up, on into the diary entries |
+| `Enter` | to the card on its own board, or open the entry |
+| `Space` | finish the card where it stands |
+| `D` | due date |
+| `H` / `L` | to the list left / right on its board |
+| `:` | the command line with the agenda kept up, so `:repeat` acts on the card |
+| `T` | today's diary entry |
+| `t`, `q`, `Esc` | close |
+
+The agenda opens by itself once a day when something is late or due. The bar
+shows the `late` and `today` counts whenever either is above zero. With mail
+set up, cards due today are mailed in the morning too; see
+[Backup and mail](Backup-and-Mail.md#mail).