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.go

@@ -0,0 +1,84 @@
+package shell
+
+import (
+	"errors"
+	"fmt"
+	"math/rand/v2"
+	"sync"
+)
+
+type registryEntry struct {
+	shell  Shell
+	weight int
+}
+
+// Registry holds shells with associated weights for random selection.
+type Registry struct {
+	mu      sync.RWMutex
+	entries []registryEntry
+}
+
+// NewRegistry returns an empty Registry.
+func NewRegistry() *Registry {
+	return &Registry{}
+}
+
+// Register adds a shell with the given weight. Weight must be >= 1 and
+// no duplicate names are allowed.
+func (r *Registry) Register(shell Shell, weight int) error {
+	if weight < 1 {
+		return fmt.Errorf("weight must be >= 1, got %d", weight)
+	}
+
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	for _, e := range r.entries {
+		if e.shell.Name() == shell.Name() {
+			return fmt.Errorf("shell %q already registered", shell.Name())
+		}
+	}
+
+	r.entries = append(r.entries, registryEntry{shell: shell, weight: weight})
+	return nil
+}
+
+// Select picks a shell using weighted random selection.
+func (r *Registry) Select() (Shell, error) {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+
+	if len(r.entries) == 0 {
+		return nil, errors.New("no shells registered")
+	}
+
+	total := 0
+	for _, e := range r.entries {
+		total += e.weight
+	}
+
+	pick := rand.IntN(total)
+	cumulative := 0
+	for _, e := range r.entries {
+		cumulative += e.weight
+		if pick < cumulative {
+			return e.shell, nil
+		}
+	}
+
+	// Should never reach here, but return last entry as fallback.
+	return r.entries[len(r.entries)-1].shell, nil
+}
+
+// Get returns a shell by name.
+func (r *Registry) Get(name string) (Shell, bool) {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+
+	for _, e := range r.entries {
+		if e.shell.Name() == name {
+			return e.shell, true
+		}
+	}
+	return nil, false
+}