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/protocol.rs

@@ -5,7 +5,12 @@ use anyhow::{Context, Result, bail};
 use serde::{Deserialize, Serialize};
 
 pub const MAGIC: &[u8; 4] = b"DOSH";
-pub const VERSION: u8 = 2;
+// v3: added `ForwardingKind::Agent` (SSH-agent forwarding). The new variant rides
+// inside `NativeUserAuth.requested_forwardings`, so a pre-agent peer would fail to
+// deserialize it; bumping the wire version makes such a peer answer with a clear
+// version-mismatch reject instead. Existing variants' bincode discriminants are
+// unchanged, so the bump is purely a compatibility gate.
+pub const VERSION: u8 = 3;
 pub const HEADER_LEN: usize = 58;
 
 /// Stable, user-facing reason string the server puts in an `AttachReject` when a