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/shell/bash/bash_test.go

@@ -0,0 +1,198 @@
+package bash
+
+import (
+	"bytes"
+	"context"
+	"io"
+	"strings"
+	"testing"
+	"time"
+
+	"git.t-juice.club/torjus/oubliette/internal/shell"
+	"git.t-juice.club/torjus/oubliette/internal/storage"
+)
+
+type rwCloser struct {
+	io.Reader
+	io.Writer
+	closed bool
+}
+
+func (r *rwCloser) Close() error {
+	r.closed = true
+	return nil
+}
+
+func TestFormatPrompt(t *testing.T) {
+	tests := []struct {
+		cwd  string
+		want string
+	}{
+		{"/root", "root@host:~# "},
+		{"/root/sub", "root@host:~/sub# "},
+		{"/tmp", "root@host:/tmp# "},
+		{"/", "root@host:/# "},
+	}
+
+	for _, tt := range tests {
+		state := &shellState{cwd: tt.cwd, username: "root", hostname: "host"}
+		got := formatPrompt(state)
+		if got != tt.want {
+			t.Errorf("formatPrompt(cwd=%q) = %q, want %q", tt.cwd, got, tt.want)
+		}
+	}
+}
+
+func TestReadLineEnter(t *testing.T) {
+	input := bytes.NewBufferString("hello\r")
+	var output bytes.Buffer
+	rw := struct {
+		io.Reader
+		io.Writer
+	}{input, &output}
+
+	ctx := context.Background()
+	line, err := readLine(ctx, rw)
+	if err != nil {
+		t.Fatalf("readLine: %v", err)
+	}
+	if line != "hello" {
+		t.Errorf("line = %q, want %q", line, "hello")
+	}
+}
+
+func TestReadLineBackspace(t *testing.T) {
+	// Type "helo", backspace, then "lo\r"
+	input := bytes.NewBuffer([]byte{'h', 'e', 'l', 'o', 127, 'l', 'o', '\r'})
+	var output bytes.Buffer
+	rw := struct {
+		io.Reader
+		io.Writer
+	}{input, &output}
+
+	ctx := context.Background()
+	line, err := readLine(ctx, rw)
+	if err != nil {
+		t.Fatalf("readLine: %v", err)
+	}
+	if line != "hello" {
+		t.Errorf("line = %q, want %q", line, "hello")
+	}
+}
+
+func TestReadLineCtrlC(t *testing.T) {
+	input := bytes.NewBuffer([]byte("partial\x03"))
+	var output bytes.Buffer
+	rw := struct {
+		io.Reader
+		io.Writer
+	}{input, &output}
+
+	ctx := context.Background()
+	line, err := readLine(ctx, rw)
+	if err != nil {
+		t.Fatalf("readLine: %v", err)
+	}
+	if line != "" {
+		t.Errorf("line after Ctrl+C = %q, want empty", line)
+	}
+}
+
+func TestReadLineCtrlD(t *testing.T) {
+	input := bytes.NewBuffer([]byte{4}) // Ctrl+D on empty line
+	var output bytes.Buffer
+	rw := struct {
+		io.Reader
+		io.Writer
+	}{input, &output}
+
+	ctx := context.Background()
+	_, err := readLine(ctx, rw)
+	if err != io.EOF {
+		t.Fatalf("expected io.EOF, got %v", err)
+	}
+}
+
+func TestBashShellHandle(t *testing.T) {
+	store := storage.NewMemoryStore()
+	sessID, _ := store.CreateSession(context.Background(), "127.0.0.1", "root", "bash")
+
+	sess := &shell.SessionContext{
+		SessionID: sessID,
+		Username:  "root",
+		Store:     store,
+		CommonConfig: shell.ShellCommonConfig{
+			Hostname: "testhost",
+			Banner:   "Welcome to Ubuntu 22.04.3 LTS\r\n\r\n",
+		},
+	}
+
+	// Simulate typing commands followed by "exit\r"
+	commands := "pwd\rwhoami\rexit\r"
+	clientInput := bytes.NewBufferString(commands)
+	var clientOutput bytes.Buffer
+	rw := &rwCloser{
+		Reader: clientInput,
+		Writer: &clientOutput,
+	}
+
+	sh := NewBashShell()
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	err := sh.Handle(ctx, sess, rw)
+	if err != nil {
+		t.Fatalf("Handle: %v", err)
+	}
+
+	output := clientOutput.String()
+
+	// Should contain banner.
+	if !strings.Contains(output, "Welcome to Ubuntu") {
+		t.Error("output should contain banner")
+	}
+
+	// Should contain prompt with hostname.
+	if !strings.Contains(output, "root@testhost") {
+		t.Errorf("output should contain prompt, got: %s", output)
+	}
+
+	// Check session logs were recorded.
+	if len(store.SessionLogs) < 2 {
+		t.Errorf("expected at least 2 session logs, got %d", len(store.SessionLogs))
+	}
+}
+
+func TestBashShellFakeUser(t *testing.T) {
+	store := storage.NewMemoryStore()
+	sessID, _ := store.CreateSession(context.Background(), "127.0.0.1", "attacker", "bash")
+
+	sess := &shell.SessionContext{
+		SessionID: sessID,
+		Username:  "attacker",
+		Store:     store,
+		CommonConfig: shell.ShellCommonConfig{
+			Hostname: "testhost",
+			FakeUser: "admin",
+		},
+	}
+
+	commands := "whoami\rexit\r"
+	clientInput := bytes.NewBufferString(commands)
+	var clientOutput bytes.Buffer
+	rw := &rwCloser{
+		Reader: clientInput,
+		Writer: &clientOutput,
+	}
+
+	sh := NewBashShell()
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	sh.Handle(ctx, sess, rw)
+
+	output := clientOutput.String()
+	if !strings.Contains(output, "admin") {
+		t.Errorf("output should contain fake user 'admin', got: %s", output)
+	}
+}