feat: add shell interface, registry, and bash shell emulator

Implement Phase 1.4: replaces the hardcoded banner/timeout stub with a
proper shell system. Adds a Shell interface with weighted registry for
shell selection, a RecordingChannel wrapper (pass-through for now, prep
for Phase 2.3 replay), and a bash-like shell with fake filesystem,
terminal line reader, and command handling (pwd, ls, cd, cat, whoami,
hostname, id, uname, exit). Sessions now log command/output pairs to
the store and record the shell name.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/server/server_test.go

@@ -1,11 +1,13 @@
 package server
 
 import (
+	"bytes"
 	"context"
 	"log/slog"
 	"net"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -109,6 +111,10 @@ func TestIntegrationSSHConnect(t *testing.T) {
 				{Username: "root", Password: "toor"},
 			},
 		},
+		Shell: config.ShellConfig{
+			Hostname: "ubuntu-server",
+			Banner:   "Welcome to Ubuntu 22.04.3 LTS\r\n\r\n",
+		},
 		LogLevel: "debug",
 	}
 
@@ -152,7 +158,7 @@ func TestIntegrationSSHConnect(t *testing.T) {
 		time.Sleep(50 * time.Millisecond)
 	}
 
-	// Test static credential login.
+	// Test static credential login with shell interaction.
 	t.Run("static_cred", func(t *testing.T) {
 		clientCfg := &ssh.ClientConfig{
 			User:            "root",
@@ -172,6 +178,62 @@ func TestIntegrationSSHConnect(t *testing.T) {
 			t.Fatalf("new session: %v", err)
 		}
 		defer session.Close()
+
+		// Request PTY and shell.
+		if err := session.RequestPty("xterm", 80, 40, ssh.TerminalModes{}); err != nil {
+			t.Fatalf("request pty: %v", err)
+		}
+
+		stdin, err := session.StdinPipe()
+		if err != nil {
+			t.Fatalf("stdin pipe: %v", err)
+		}
+
+		var output bytes.Buffer
+		session.Stdout = &output
+
+		if err := session.Shell(); err != nil {
+			t.Fatalf("shell: %v", err)
+		}
+
+		// Wait for the prompt, then send commands.
+		time.Sleep(500 * time.Millisecond)
+		stdin.Write([]byte("pwd\r"))
+		time.Sleep(200 * time.Millisecond)
+		stdin.Write([]byte("whoami\r"))
+		time.Sleep(200 * time.Millisecond)
+		stdin.Write([]byte("exit\r"))
+
+		// Wait for session to end.
+		session.Wait()
+
+		out := output.String()
+		if !strings.Contains(out, "Welcome to Ubuntu") {
+			t.Errorf("output should contain banner, got: %s", out)
+		}
+		if !strings.Contains(out, "/root") {
+			t.Errorf("output should contain /root from pwd, got: %s", out)
+		}
+		if !strings.Contains(out, "root") {
+			t.Errorf("output should contain 'root' from whoami, got: %s", out)
+		}
+
+		// Verify session logs were recorded.
+		if len(store.SessionLogs) < 2 {
+			t.Errorf("expected at least 2 session logs, got %d", len(store.SessionLogs))
+		}
+
+		// Verify session was created with shell name.
+		var foundBash bool
+		for _, s := range store.Sessions {
+			if s.ShellName == "bash" {
+				foundBash = true
+				break
+			}
+		}
+		if !foundBash {
+			t.Error("expected a session with shell_name='bash'")
+		}
 	})
 
 	// Test wrong password is rejected.