Stop the docs claiming absences that have since been built

An audit of README.md, the seven docs and the fourteen ADRs against the code, looking for what is described
as absent or planned. Most of it held. What did not is here, and it clusters: every stale claim but one is
downstream of the settings file arriving without this document noticing.

design-import-gaps said the client has no preferences store and writes exactly two files. It writes three —
ClientSettings is in settings.json beside the cache — and two preferences are saved through it. From that
one error followed four more: the terminal font size row said "fixed at the renderer's 13px" when it has
been 8 to 32 from a screen and three chords for some time; the transfer-resume row and the per-host last
directory row both blamed a store that now exists, when what they actually want is a table and a scalar file
is the wrong shape for one; and the Preferences table asserted no preference could be saved at all.

It also said TerminalServerOpcode has four values and none carries an option. It has eight, and one of them
is FontSize — which is the interesting part rather than a counting error, because that opcode is the proof
that the frame these rows say is missing can be built. The rows now say what each one would actually take,
which for three of the four is a setting, an opcode and a control, and for the Backspace row is a reason:
which byte backspace sends is a fact about the remote's stty, so a client-side switch fixes a mismatch by
hiding it.

THE SAME TWO ERRORS WERE SHIPPED IN THE INTERFACE. The preferences screen carries a NOT BUILT YET list, so
that what the screen does not do is as legible as what it does — and it said terminal size was hard-coded a
hundred lines below a working size control, and said there is one release channel a month after the nightly
shipped. A list of absences is only worth having if it is true, and a screen contradicting itself in the
same scroll is worse than no list. Both lines are corrected rather than removed: the first now says which
three of the four are genuinely hard-coded, and the second says what is actually missing, which is a way to
change channel from inside the application rather than by installing the other build.

docs/adding-hosts-on-the-phone.md is deleted. It was a work plan whose own header says "Status: built. All
six steps." — nothing links to it, and the decisions it records are in the code it produced, including the
one it is proudest of: HostSecret.AsksForPassword carries its own remark on why naming neither binding had
to stop meaning "ask me". What was left was step ordering and per-test instructions for work that shipped.
Git keeps it.

crypto.md is deliberately untouched. It is normative and frozen, and its claims are about the DSH1 format
rather than about this build — including the one that reads oddly next to the code, that a passphrase is
one of four ways to open a vault. Under the spec it is. What is missing is a statement about what this
build can open, and that belongs beside the spec rather than inside it; see
docs/unlocking-without-the-passphrase.md.

VERIFIED. Build clean, 112 layout tests, 354 app tests. The preferences screen is measured by the layout
suite, so the longer copy is held inside the window at the minimum size rather than assumed to fit.

src/DodoSSH.Client.App/Views/PreferencesScreen.axaml

@@ -256,9 +256,9 @@
           </Style>
         </ItemsControl.Styles>
         <TextBlock Classes="gap"
-                   Text="Terminal font, size, cursor and scrollback — the renderer hard-codes them, and nothing carries a change to it." />
+                   Text="Terminal font, cursor and scrollback — the renderer hard-codes those three. The text size above is the one that is not, and it is the path the others would follow if they were worth a row here." />
         <TextBlock Classes="gap"
-                   Text="A beta channel — there is one release channel, and a switch offering a second would be a preference with nothing behind it." />
+                   Text="Changing channel from in here — there is a nightly as well as a release, but which one a copy follows is fixed when it is built, so moving between them means installing the other one." />
         <TextBlock Classes="gap"
                    Text="Auto-lock after idle — nothing tracks idleness, and the lock policy would have to decide what to do about a shell mid-job." />
         <TextBlock Classes="gap"