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/config/config_test.go

@@ -169,6 +169,59 @@ retention_interval = "2h"
 	}
 }
 
+func TestLoadShellDefaults(t *testing.T) {
+	path := writeTemp(t, "")
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if cfg.Shell.Hostname != "ubuntu-server" {
+		t.Errorf("default hostname = %q, want %q", cfg.Shell.Hostname, "ubuntu-server")
+	}
+	if cfg.Shell.Banner == "" {
+		t.Error("default banner should not be empty")
+	}
+	if cfg.Shell.FakeUser != "" {
+		t.Errorf("default fake_user = %q, want empty", cfg.Shell.FakeUser)
+	}
+}
+
+func TestLoadShellConfig(t *testing.T) {
+	content := `
+[shell]
+hostname = "myhost"
+banner = "Custom banner\r\n"
+fake_user = "admin"
+
+[shell.bash]
+custom_key = "value"
+`
+	path := writeTemp(t, content)
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if cfg.Shell.Hostname != "myhost" {
+		t.Errorf("hostname = %q, want %q", cfg.Shell.Hostname, "myhost")
+	}
+	if cfg.Shell.Banner != "Custom banner\r\n" {
+		t.Errorf("banner = %q, want %q", cfg.Shell.Banner, "Custom banner\r\n")
+	}
+	if cfg.Shell.FakeUser != "admin" {
+		t.Errorf("fake_user = %q, want %q", cfg.Shell.FakeUser, "admin")
+	}
+	if cfg.Shell.Shells == nil {
+		t.Fatal("Shells map should not be nil")
+	}
+	bashCfg, ok := cfg.Shell.Shells["bash"]
+	if !ok {
+		t.Fatal("Shells[\"bash\"] not found")
+	}
+	if bashCfg["custom_key"] != "value" {
+		t.Errorf("Shells[\"bash\"][\"custom_key\"] = %v, want %q", bashCfg["custom_key"], "value")
+	}
+}
+
 func TestLoadMissingFile(t *testing.T) {
 	_, err := Load("/nonexistent/path/config.toml")
 	if err == nil {