Let a shared vault arrive, a bucket be found, and a vault be deleted

Three things a user reported, one of which was a real bug and one of which was
not the bug it looked like.

**A vault shared with somebody never reached their machine.** The grant was
correct at both ends: the sharing client verified the recipient's key against the
key log and wrapped every generation to it, the server stored it, and /me would
have returned it. Nothing asked. VaultSession.RefreshVaultsAsync — the method
whose own summary says it is "called after a share and on a periodic pass" — had
no caller anywhere in the application, so the vault list was whatever the last
browser sign-in cached. A restart did not help: an offline unlock reads that same
cache. The vault appeared only if the recipient happened to sign in through the
browser again, which is why this looked like sharing being broken rather than
like a list that was never re-read.

So every synchronisation pass now re-reads it, before it syncs. SyncOnceAsync
takes the whole server rather than its sync half for that reason, and the order
matters: a vault admitted by the refresh is one that same pass then pulls, where
the other order would show a newly shared vault as an empty one until the minute
after. The shell is told only when the set actually changed — it rebuilds the tab
strip's vault menu from the session's list, and doing that on every quiet pass
would rebuild a menu once a minute for nothing.

The test needed the fake server to be able to do something no test here had
needed before: hand this account a vault it did not make. ShareVaultWithMe wraps
a real key to the encryption key this account enrolled, so the keyring opens it
exactly as it opens a real colleague's — a helper that filled the field with
bytes would let a vault appear in the list and never prove it could be read.

**Adding an S3 bucket on the desktop works, and could not be found.** The report
was that it is not possible; driving the real XAML headlessly says otherwise —
Keychain, + BUCKET, and the editor saves. What is true is that S3 is where
somebody goes looking, and from there SELECT BUCKET opened a combo box with
nothing in it and no sentence anywhere saying that a bucket is a keychain item.
From where the user was standing that is indistinguishable from an application
with no way to add one.

The empty state now says what a bucket is and offers a button that lands on the
keychain with the editor already open — navigating to the screen and leaving
+ BUCKET to be found among five buttons would be most of the same problem. The
phone gets the sentence and no button: its keychain screen reads and deletes and
edits nothing, so there is no editor to send anybody to, and naming the machine
that has one beats an empty control that reads as a screen still loading.

The keychain screen's layout test grew the two categories it never covered.
Tags and buckets arrived after it was written, and the header strip it measures
is one that has overflowed twice before.

**A vault can now be deleted.** DELETE /api/v1/vaults/{id}, gated on Admin —
the line the rename already drew, for a stronger version of its reason, since
this takes the vault from everybody in it at once. The row is soft-deleted and
every grant to it withdrawn in one write; VaultAccessService filters on the stamp
at both ends, so from that moment the vault is absent from every member's /me and
every call naming it answers 404. Their clients notice on the pass described
above.

The team behind it is archived when it owned nothing else, which is the mirror of
renaming it: a vault made from the vaults screen gets a team named after it that
nobody was ever shown, and leaving that behind would leave a membership list no
screen has a row for. That is a second call rather than one transaction —
archiving is TeamService's, it refuses while a team owns vaults, and it can only
tell that this one no longer does once the deletion is committed. A crash between
the two leaves an empty team: invisible, archivable afterwards, harmless, and a
better failure than a vault that could not be deleted because tidying up after it
did not work.

Two refusals worth stating. The personal vault cannot be deleted at either end:
it is created by enrollment, everything filed nowhere else lives in it, and no
call would make another. And the items are kept — ciphertext behind a vault
nothing will resolve, so deleting them buys no confidentiality while destroying
what an operator undoing a mistake would need.

The client drops the key from the keyring and the row from the cache rather than
waiting for a refresh, so the list is right immediately; the items stay, as they
stay for a vault whose grant was withdrawn, because a copy is on every other
member's machine too and removing these rows would be the client pretending to a
reach it does not have. The confirmation says that out loud before it is
answered. It is the one sentence this screen must not leave implied: deletion is
no more retroactive than revocation is. See ADR 0001.

Desktop only, deliberately. The Android vaults screen offers no rename and no
hand-over either, so adding delete alone there would be the one destructive vault
operation on a screen with no other.

Three places asserted that a vault can never be deleted — TeamService's refusal
message, the TeamNotEmpty problem code, and ADR 0009 — and each now names the
route instead.

README.md

@@ -307,10 +307,12 @@ usually staying in it.
 
 Six limits, stated rather than discovered:
 
-- **A vault cannot be deleted.** Nothing in this product removes one, and the server refuses to archive the
-  membership list behind a vault that still exists — a shared vault is readable *because* of membership, so
-  archiving it would take the vault away from everybody holding a key, including you, quietly and all at
-  once. The screen says so where you would otherwise go looking for the button.
+- **Deleting a vault does not reach a machine that has already synced it.** An admin or the owner can
+  delete a shared vault: it leaves everybody's list at once, every key to it is withdrawn, and the
+  membership list behind it is archived when it existed to carry that vault alone. What it cannot do is
+  take back the copy a colleague pulled yesterday — the same limit revocation has, for the same reason —
+  and the question you are asked before it happens says so. Your personal vault is refused: everything
+  filed nowhere else lives in it, and there is no way to make another.
 - **Removing a member is not retroactive.** It revokes their grants, rotates every vault behind that
   membership list your machine can open, and hands each new key to the members who are left — so nothing
   written from that point on is readable to them. Everything they already pulled is still on their machine.
@@ -622,8 +624,8 @@ keychain plus a terminal — and the spike that gates all of it.
   membership to permissions, so a viewer may pull and may not push; the desktop client reads and syncs
   every vault it holds a key for, and a real VAULTS screen replaces the placeholder. The screen is
   vault-shaped rather than team-shaped: naming a vault makes the membership list that carries it, so the
-  team is behind the vault rather than a thing anybody has to create first. A vault can be renamed and
-  handed to another member; a member row carries when that account was last here; and an address with no
+  team is behind the vault rather than a thing anybody has to create first. A vault can be renamed, deleted
+  and handed to another member; a member row carries when that account was last here; and an address with no
   account on this deployment can be invited, joining the moment somebody signs in with it. See
   [Sharing a vault](#sharing-a-vault) for the one distinction the whole design rests on, and the limits worth
   knowing before you rely on it; the reasoning is in
@@ -639,6 +641,11 @@ keychain plus a terminal — and the spike that gates all of it.
   the rotation is re-sealed as it is pushed, so nothing reaches the server under a superseded key at all.
   See [ADR 0010](docs/adr/0010-vault-key-rotation.md).
 
+  **A vault shared with you arrives on the next synchronisation pass**, within the minute, with no sign-in
+  and nothing to press. There is no push channel, so each pass asks the server which vaults this account can
+  reach before syncing the ones it already knows — which is also how a vault that has been deleted, or one
+  whose grant was withdrawn, stops being listed.
+
   **Ownership transfer is here, and it is one write rather than two.** The member you name becomes owner
   and you become an admin, in a single transaction — because ownership is sole, so promoting first leaves
   the team owned twice and demoting first leaves it owned by nobody, and there is nobody left with the