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/recorder_test.go

@@ -0,0 +1,43 @@
+package shell
+
+import (
+	"bytes"
+	"io"
+	"testing"
+)
+
+// nopCloser wraps a ReadWriter with a no-op Close.
+type nopCloser struct {
+	io.ReadWriter
+}
+
+func (nopCloser) Close() error { return nil }
+
+func TestRecordingChannelPassthrough(t *testing.T) {
+	var buf bytes.Buffer
+	rc := NewRecordingChannel(nopCloser{&buf})
+
+	// Write through the recorder.
+	msg := []byte("hello")
+	n, err := rc.Write(msg)
+	if err != nil {
+		t.Fatalf("Write: %v", err)
+	}
+	if n != len(msg) {
+		t.Errorf("Write n = %d, want %d", n, len(msg))
+	}
+
+	// Read through the recorder.
+	out := make([]byte, 16)
+	n, err = rc.Read(out)
+	if err != nil {
+		t.Fatalf("Read: %v", err)
+	}
+	if string(out[:n]) != "hello" {
+		t.Errorf("Read = %q, want %q", out[:n], "hello")
+	}
+
+	if err := rc.Close(); err != nil {
+		t.Fatalf("Close: %v", err)
+	}
+}