Share a vault with a team, without the server holding a key

M3's teams, sharing and ACLs. Teams with roles, a public-key directory, the
append-only key log served for clients to check it against, team-owned vaults,
and vault key grants wrapped by a client and stored opaquely by the server.
VaultAccessService resolves team membership to PermissionFlags, 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 TEAMS screen replaces the one that said it did not exist.
No migration: team, team_membership, vault.team_id and vault_key_grant have all
been there since the first one, which is what carrying two unused tables bought.

Membership is authorisation. A grant is access. The obvious model is one
concept — "access", with a role attached, handed out by the server — and this
architecture cannot implement it: a vault key is sealed to each member's X25519
key, and only a client holding the plaintext can seal it for somebody else. So
"give Bob access" decomposes into a database write and a wrap, which happen on
different machines. Adding a member makes the server serve them the vault; it
cannot make it readable. VaultSummary.WrappedVaultKey is null in the meantime
and the vault appears in their list saying it is waiting for a key, because
hiding it until a grant existed would have been tidier and would have implied
the server was the thing granting access. The screen says the same thing after
every add, in the status line. ADR 0009 records the whole decision.

Sharing verifies or refuses. A directory lookup is a claim by the server about
a third party's public key, and wrapping to an unverified claim hands the vault
to whoever made it — no amount of transport security helps, because the server
is inside the threat model. KeyLogAudit reads the whole log, recomputes every
entry's hash from its own contents, checks the chain from genesis, and refuses
unless the offered key appears in it unchanged. There is no override flag: one
that exists gets used on the day the log is briefly unreachable, and the
resulting grant is indistinguishable from a correct one afterwards. What it
still cannot promise is that the key is the right person's, so the fingerprint
comes back for an out-of-band comparison and the success message says so every
time. A test corrupts the fake server's log by one byte and watches the client
refuse rather than warn.

The roles are only the ones that are enforceable. There is no ConnectOnly,
despite the design asking for one and TeamRole having room: SSH terminates on
the client, so a session needs the credential's plaintext on that machine, and
"may connect but may not read the key" cannot be enforced here. Shipping it as
an option in a dropdown would have been a lie. Connect rides along with Read
and is documented as an interface hint. Removal is named for what it does — it
revokes grants and flags the vault for rekey, and claims nothing about what is
already on somebody's laptop.

Three things are deliberately absent, and each is a refusal rather than an
omission. The rekey itself, because re-wrapping every item's data key under a
new vault key needs a client holding the current one; the server records that a
rotation is owed and the interface reports it, which is more honest than a
button that only appears to do it. Ownership transfer, because allowing an
owner to be removed without one leaves a team nobody can administer. And
cross-vault host key trust: a pin in a team vault is listed but not consulted
at connect time, because any member with Write could otherwise pre-approve a
fingerprint another member's client then trusts silently for a host in their
own vault. Scoping trust properly needs a scope on the SSH connect path, which
IKnownHostStore has not got; until then the narrow direction is the safe one
and the cost is in the README rather than hidden.

Reading now spans vaults and writing still does not. Every list on the vault
and hosts screens covers each vault the keyring opened, rows carry the vault
they came from, and an edit goes back to that vault rather than to the active
one — writing it to the active vault would fork the item and only show up when
a colleague wondered why their change never arrived. A new item goes wherever a
picker says, defaulting to the personal vault and never moving on its own,
because an item filed into a team's vault is visible to that team and moving it
back means deleting and retyping. The sidebar heading stops naming one vault
once there are two, and each row names its own.

The server checks what it can and nothing it cannot. It will not record a grant
for a key its recipient no longer holds, for a superseded generation, or for
somebody who is not in the team — each of those would otherwise surface days
later at the far end as a tag failure indistinguishable from corruption. It
does not verify the wrap or the signature, and the grant service says so: that
would be a convenience and never the boundary, and would put an asymmetric
implementation on a machine that is supposed to hold no keys.

Two bugs the tests found. TeamsViewModel's busy gate blocked its own reload, so
a team created a moment earlier was missing from the list it had just been
added to. And syncing every vault turned a failure from an exception into a
report, which made a background pass announce an unreachable vault once a
minute — the exact behaviour AnAutomaticPassThatFails_LeavesTheStatusAlone
exists to prevent. The fact is recorded and the message swallowed, as it was
before; pressing Sync still names the vault and the reason.

Also fixes a build break this branch started with: QuickConnectTests was never
updated when M2 added ISftpSessionFactory to the shell's constructor, so
nothing built at all.

src/DodoSSH.Client.App/Views/TeamsScreen.axaml.cs

@@ -0,0 +1,9 @@
+using Avalonia.Controls;
+
+namespace DodoSSH.Client.App.Views;
+
+/// <summary>Teams: who is in one, what they may do, and which vaults they hold a key to.</summary>
+internal sealed partial class TeamsScreen : UserControl
+{
+    public TeamsScreen() => InitializeComponent();
+}