fix: gate SSH agent auth behind #[cfg(unix)] for Windows compat

AgentClient::connect_env() uses Unix domain sockets and is unavailable
on Windows. Add a #[cfg(not(unix))] stub that returns an error message.

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Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

src-tauri/src/ssh.rs

@@ -203,6 +203,8 @@ impl SshConnectionPool {
     }
 
     /// Try all keys offered by the ssh-agent until one succeeds.
+    /// Only available on Unix (SSH agent uses a Unix domain socket).
+    #[cfg(unix)]
     async fn authenticate_with_agent(
         &self,
         session: &mut client::Handle<SshHandler>,
@@ -240,6 +242,15 @@ impl SshConnectionPool {
         Ok(false)
     }
 
+    #[cfg(not(unix))]
+    async fn authenticate_with_agent(
+        &self,
+        _session: &mut client::Handle<SshHandler>,
+        _username: &str,
+    ) -> Result<bool, String> {
+        Err("SSH agent forwarding is not supported on Windows".into())
+    }
+
     // -- resolve_home -----------------------------------------------------
 
     /// Run `echo $HOME` over a fresh channel to discover the remote home dir.