SSH connections, kept by the people who own them

An agent chat will act on a machine you choose, so this is the screen where
you choose it. User-owned like a note, not admin-owned like a connection:
these are somebody's own machines and somebody's own keys, and "anyone in
this group may log in to my server" is a different feature with a different
blast radius. services/sharing.py is deliberately not involved either --
sharing grants reading, and a host somebody else can read is a host they
can log in to.

Trust on first use, made explicit rather than assumed. Adding a host does
not connect to it. Check looks at its key and shows you the fingerprint;
nothing is sent until you accept, because get_server_host_key completes the
key exchange and stops -- no username, no credential. Accepting pins it,
and a host that later presents a different key is refused with the reason
rather than quietly trusted. Moving a profile to another host or port
forgets the pin, since a key belongs to the machine it came from.

Four asyncssh defaults are actively wrong here and all four are passed
explicitly: every LLeMbas user shares one unix account, so `known_hosts`
would be a shared trust store, `client_keys` would authenticate one person
with another's key, `config` would let a ProxyCommand redirect the
connection, and `agent_path` would silently use $SSH_AUTH_SOCK. There is a
test for exactly that, and it needs no server.

Files go over SFTP rather than through a shell. The SSH exec protocol
carries one command *string* that the far side parses, with no argv form at
all, so a model-supplied path in a command line is unavoidably a quoting
problem. Over SFTP a path is a path.

Chat gains its kind, connection, project directory and mode; the first
three are fixed once a chat has a message, because a transcript whose
earlier turns ran somewhere else is not one conversation.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/web/static/css/admin.css

@@ -471,3 +471,9 @@ a.tabs__tab { text-decoration: none; }
   background: var(--code-bg);
   border: 1px solid var(--code-border);
 }
+
+/* --- The permission modes, explained on the agents page ------------------- */
+.mode-list { margin: 0; display: flex; flex-direction: column; gap: var(--sp-2); }
+.mode-list__row { display: flex; gap: var(--sp-3); align-items: baseline; }
+.mode-list__row dt { flex: 0 0 5rem; color: var(--ink); }
+.mode-list__row dd { margin: 0; color: var(--ink-muted); font-size: var(--text-sm); }