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/shell.go

@@ -0,0 +1,33 @@
+package shell
+
+import (
+	"context"
+	"io"
+
+	"git.t-juice.club/torjus/oubliette/internal/storage"
+)
+
+// Shell is the interface that all honeypot shell implementations must satisfy.
+type Shell interface {
+	Name() string
+	Description() string
+	Handle(ctx context.Context, sess *SessionContext, rw io.ReadWriteCloser) error
+}
+
+// SessionContext carries metadata about the current SSH session.
+type SessionContext struct {
+	SessionID     string
+	Username      string
+	RemoteAddr    string
+	ClientVersion string
+	Store         storage.Store
+	ShellConfig   map[string]any
+	CommonConfig  ShellCommonConfig
+}
+
+// ShellCommonConfig holds settings shared across all shell types.
+type ShellCommonConfig struct {
+	Hostname string
+	Banner   string
+	FakeUser string // override username in prompt; empty = use authenticated user
+}