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/config/config.go

@@ -9,11 +9,19 @@ import (
 )
 
 type Config struct {
-	SSH      SSHConfig      `toml:"ssh"`
-	Auth     AuthConfig     `toml:"auth"`
-	Storage  StorageConfig  `toml:"storage"`
-	LogLevel  string `toml:"log_level"`
-	LogFormat string `toml:"log_format"` // "text" (default) or "json"
+	SSH       SSHConfig      `toml:"ssh"`
+	Auth      AuthConfig     `toml:"auth"`
+	Storage   StorageConfig  `toml:"storage"`
+	Shell     ShellConfig    `toml:"shell"`
+	LogLevel  string         `toml:"log_level"`
+	LogFormat string         `toml:"log_format"` // "text" (default) or "json"
+}
+
+type ShellConfig struct {
+	Hostname string                    `toml:"hostname"`
+	Banner   string                    `toml:"banner"`
+	FakeUser string                    `toml:"fake_user"`
+	Shells   map[string]map[string]any `toml:"-"` // per-shell config extracted manually
 }
 
 type StorageConfig struct {
@@ -56,6 +64,14 @@ func Load(path string) (*Config, error) {
 		return nil, fmt.Errorf("parsing config: %w", err)
 	}
 
+	// Second pass: extract per-shell sub-tables (e.g. [shell.bash]).
+	var raw map[string]any
+	if err := toml.Unmarshal(data, &raw); err == nil {
+		if shellSection, ok := raw["shell"].(map[string]any); ok {
+			cfg.Shell.Shells = extractShellTables(shellSection)
+		}
+	}
+
 	applyDefaults(cfg)
 
 	if err := validate(cfg); err != nil {
@@ -96,6 +112,36 @@ func applyDefaults(cfg *Config) {
 	if cfg.Storage.RetentionInterval == "" {
 		cfg.Storage.RetentionInterval = "1h"
 	}
+	if cfg.Shell.Hostname == "" {
+		cfg.Shell.Hostname = "ubuntu-server"
+	}
+	if cfg.Shell.Banner == "" {
+		cfg.Shell.Banner = "Welcome to Ubuntu 22.04.3 LTS (GNU/Linux 5.15.0-89-generic x86_64)\r\n\r\n"
+	}
+}
+
+// knownShellKeys are top-level keys in [shell] that are not per-shell sub-tables.
+var knownShellKeys = map[string]bool{
+	"hostname":  true,
+	"banner":    true,
+	"fake_user": true,
+}
+
+// extractShellTables pulls per-shell config sub-tables from the raw [shell] section.
+func extractShellTables(section map[string]any) map[string]map[string]any {
+	result := make(map[string]map[string]any)
+	for key, val := range section {
+		if knownShellKeys[key] {
+			continue
+		}
+		if sub, ok := val.(map[string]any); ok {
+			result[key] = sub
+		}
+	}
+	if len(result) == 0 {
+		return nil
+	}
+	return result
 }
 
 func validate(cfg *Config) error {