Add opt-in, security-gated SSH-agent forwarding

Item 3: SSH-agent forwarding, double-gated (client -A/forward_agent AND server
allow_agent_forwarding) and off by default on both ends.

Wire: new ForwardingKind::Agent (appended, existing bincode discriminants
unchanged). Bumped protocol VERSION 2 -> 3 so a pre-agent peer answers with a
clear version-mismatch reject instead of a deserialize error.

Client: -A / --forward-agent flag; requires local SSH_AUTH_SOCK. Requests an
Agent forwarding during native auth (agent forwarding requires native auth) and,
on a server-initiated StreamOpen carrying the reserved @dosh-agent sentinel,
splices the stream into the local agent unix socket (separate agent_writers map,
TCP forwarding paths untouched). Rejects agent StreamOpens unless it opted in, so
a server cannot reach the agent without consent.

Server: when the client opted in and allow_agent_forwarding is set, binds a
per-session proxy unix socket (dir 0700, socket 0600) before the shell spawns,
exports its path as the session's SSH_AUTH_SOCK, and tunnels each connection back
to the client over the agent sentinel stream. Applies only to freshly spawned
shells (documented). Socket cleaned up on auth/forward failure and when the
accept loop ends. SECURITY rationale documented in comments.

Tests: 2 end-to-end integration tests (full chain identities round-trip via a
PTY-hosted client + looping fake agent; and "no -A => no proxy socket"). Docs
updated (README forwarding + disconnect-status note, spec §12.1). fmt + full test
suite green (122 tests).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/native.rs

@@ -91,6 +91,12 @@ pub enum ForwardingKind {
     Local,
     Remote,
     Dynamic,
+    /// SSH-agent forwarding: the client opts in and, if the server policy allows
+    /// (`allow_agent_forwarding`), the server exports a proxy unix socket as
+    /// `SSH_AUTH_SOCK` in the remote session and tunnels each connection back to
+    /// the client's local agent over a Dosh stream. Added at the end of the enum
+    /// so bincode discriminants for the existing variants are unchanged.
+    Agent,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
@@ -513,6 +519,10 @@ impl AuthorizedKey {
                             "authorized key permitopen= cannot authorize remote or dynamic forwarding"
                         );
                     }
+                    // Agent forwarding does not open a host:port, so permitopen
+                    // (which restricts which targets may be opened) does not apply;
+                    // it is gated separately by the server's allow_agent_forwarding.
+                    ForwardingKind::Agent => {}
                 }
             }
         }