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/registry_test.go

@@ -0,0 +1,107 @@
+package shell
+
+import (
+	"context"
+	"io"
+	"testing"
+)
+
+// stubShell implements Shell for testing.
+type stubShell struct {
+	name string
+}
+
+func (s *stubShell) Name() string        { return s.name }
+func (s *stubShell) Description() string { return "stub" }
+func (s *stubShell) Handle(_ context.Context, _ *SessionContext, _ io.ReadWriteCloser) error {
+	return nil
+}
+
+func TestRegistryRegisterAndGet(t *testing.T) {
+	r := NewRegistry()
+	sh := &stubShell{name: "test"}
+
+	if err := r.Register(sh, 1); err != nil {
+		t.Fatalf("Register: %v", err)
+	}
+
+	got, ok := r.Get("test")
+	if !ok {
+		t.Fatal("Get returned false")
+	}
+	if got.Name() != "test" {
+		t.Errorf("Name = %q, want %q", got.Name(), "test")
+	}
+}
+
+func TestRegistryGetMissing(t *testing.T) {
+	r := NewRegistry()
+	_, ok := r.Get("nope")
+	if ok {
+		t.Fatal("Get returned true for missing shell")
+	}
+}
+
+func TestRegistryDuplicateName(t *testing.T) {
+	r := NewRegistry()
+	r.Register(&stubShell{name: "dup"}, 1)
+	err := r.Register(&stubShell{name: "dup"}, 1)
+	if err == nil {
+		t.Fatal("expected error for duplicate name")
+	}
+}
+
+func TestRegistryInvalidWeight(t *testing.T) {
+	r := NewRegistry()
+	err := r.Register(&stubShell{name: "a"}, 0)
+	if err == nil {
+		t.Fatal("expected error for weight 0")
+	}
+	err = r.Register(&stubShell{name: "b"}, -1)
+	if err == nil {
+		t.Fatal("expected error for negative weight")
+	}
+}
+
+func TestRegistrySelectEmpty(t *testing.T) {
+	r := NewRegistry()
+	_, err := r.Select()
+	if err == nil {
+		t.Fatal("expected error from empty registry")
+	}
+}
+
+func TestRegistrySelectSingle(t *testing.T) {
+	r := NewRegistry()
+	r.Register(&stubShell{name: "only"}, 1)
+
+	for range 10 {
+		sh, err := r.Select()
+		if err != nil {
+			t.Fatalf("Select: %v", err)
+		}
+		if sh.Name() != "only" {
+			t.Errorf("Name = %q, want %q", sh.Name(), "only")
+		}
+	}
+}
+
+func TestRegistrySelectWeighted(t *testing.T) {
+	r := NewRegistry()
+	r.Register(&stubShell{name: "heavy"}, 100)
+	r.Register(&stubShell{name: "light"}, 1)
+
+	counts := map[string]int{}
+	for range 1000 {
+		sh, err := r.Select()
+		if err != nil {
+			t.Fatalf("Select: %v", err)
+		}
+		counts[sh.Name()]++
+	}
+
+	// "heavy" has weight 100 vs "light" weight 1, so heavy should get ~99%.
+	if counts["heavy"] < 900 {
+		t.Errorf("heavy selected %d/1000 times, expected >900", counts["heavy"])
+	}
+}