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/filesystem_test.go

@@ -0,0 +1,140 @@
+package bash
+
+import (
+	"sort"
+	"testing"
+)
+
+func TestNewFilesystem(t *testing.T) {
+	fs := newFilesystem("testhost")
+
+	// Standard directories should exist.
+	for _, dir := range []string{"/etc", "/root", "/home", "/var/log", "/tmp", "/usr/bin"} {
+		if !fs.isDirectory(dir) {
+			t.Errorf("%s should be a directory", dir)
+		}
+	}
+
+	// Standard files should exist.
+	for _, file := range []string{"/etc/passwd", "/etc/hostname", "/root/.bashrc", "/tmp/notes.txt"} {
+		if !fs.exists(file) {
+			t.Errorf("%s should exist", file)
+		}
+	}
+}
+
+func TestFilesystemHostname(t *testing.T) {
+	fs := newFilesystem("myhost")
+	content, err := fs.read("/etc/hostname")
+	if err != nil {
+		t.Fatalf("read /etc/hostname: %v", err)
+	}
+	if content != "myhost\n" {
+		t.Errorf("hostname content = %q, want %q", content, "myhost\n")
+	}
+}
+
+func TestResolvePath(t *testing.T) {
+	tests := []struct {
+		cwd  string
+		arg  string
+		want string
+	}{
+		{"/root", "file.txt", "/root/file.txt"},
+		{"/root", "/etc/passwd", "/etc/passwd"},
+		{"/root", "..", "/"},
+		{"/var/log", "../..", "/"},
+		{"/root", ".", "/root"},
+		{"/root", "./sub/file", "/root/sub/file"},
+		{"/", "etc", "/etc"},
+	}
+	for _, tt := range tests {
+		got := resolvePath(tt.cwd, tt.arg)
+		if got != tt.want {
+			t.Errorf("resolvePath(%q, %q) = %q, want %q", tt.cwd, tt.arg, got, tt.want)
+		}
+	}
+}
+
+func TestFilesystemList(t *testing.T) {
+	fs := newFilesystem("testhost")
+
+	names, err := fs.list("/etc")
+	if err != nil {
+		t.Fatalf("list /etc: %v", err)
+	}
+	sort.Strings(names)
+
+	// Should contain at least passwd, hostname, hosts.
+	found := map[string]bool{}
+	for _, n := range names {
+		found[n] = true
+	}
+	for _, want := range []string{"passwd", "hostname", "hosts"} {
+		if !found[want] {
+			t.Errorf("list /etc missing %q, got %v", want, names)
+		}
+	}
+}
+
+func TestFilesystemListNonexistent(t *testing.T) {
+	fs := newFilesystem("testhost")
+	_, err := fs.list("/nonexistent")
+	if err == nil {
+		t.Fatal("expected error listing nonexistent directory")
+	}
+}
+
+func TestFilesystemListFile(t *testing.T) {
+	fs := newFilesystem("testhost")
+	_, err := fs.list("/etc/passwd")
+	if err == nil {
+		t.Fatal("expected error listing a file")
+	}
+}
+
+func TestFilesystemRead(t *testing.T) {
+	fs := newFilesystem("testhost")
+	content, err := fs.read("/etc/passwd")
+	if err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	if content == "" {
+		t.Error("expected non-empty content")
+	}
+}
+
+func TestFilesystemReadNonexistent(t *testing.T) {
+	fs := newFilesystem("testhost")
+	_, err := fs.read("/no/such/file")
+	if err == nil {
+		t.Fatal("expected error for nonexistent file")
+	}
+}
+
+func TestFilesystemReadDirectory(t *testing.T) {
+	fs := newFilesystem("testhost")
+	_, err := fs.read("/etc")
+	if err == nil {
+		t.Fatal("expected error for reading a directory")
+	}
+}
+
+func TestFilesystemDirectoryListing(t *testing.T) {
+	fs := newFilesystem("testhost")
+	names, err := fs.list("/")
+	if err != nil {
+		t.Fatalf("list /: %v", err)
+	}
+
+	// Root directories should end with /
+	found := map[string]bool{}
+	for _, n := range names {
+		found[n] = true
+	}
+	for _, want := range []string{"etc/", "root/", "home/", "var/", "tmp/", "usr/"} {
+		if !found[want] {
+			t.Errorf("list / missing %q, got %v", want, names)
+		}
+	}
+}