feat: add Smart Fridge shell and per-credential shell routing

Implement Samsung FridgeOS-themed shell (PLAN.md §3.3) with inventory
management, temperature controls, diagnostics, alerts, and other
appliance commands. Add per-credential shell routing so static
credentials can specify which shell to use via the `shell` config field,
passed through ssh.Permissions.Extensions.

Also extract shared ReadLine helper from bash to the shell package so
both shells can reuse terminal input handling.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/config/config.go

@@ -59,6 +59,7 @@ type AuthConfig struct {
 type Credential struct {
 	Username string `toml:"username"`
 	Password string `toml:"password"`
+	Shell    string `toml:"shell"` // optional: route to specific shell (empty = random)
 }
 
 type DetectionConfig struct {