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.go

@@ -0,0 +1,19 @@
+package shell
+
+import "io"
+
+// RecordingChannel wraps an io.ReadWriteCloser. In Phase 1.4 it is a
+// pass-through; Phase 2.3 will add byte-level keystroke recording here
+// without changing any shell code.
+type RecordingChannel struct {
+	inner io.ReadWriteCloser
+}
+
+// NewRecordingChannel returns a RecordingChannel wrapping rw.
+func NewRecordingChannel(rw io.ReadWriteCloser) *RecordingChannel {
+	return &RecordingChannel{inner: rw}
+}
+
+func (r *RecordingChannel) Read(p []byte) (int, error)  { return r.inner.Read(p) }
+func (r *RecordingChannel) Write(p []byte) (int, error) { return r.inner.Write(p) }
+func (r *RecordingChannel) Close() error                { return r.inner.Close() }