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>

PLAN.md

@@ -155,12 +155,12 @@ Goal: Add the entertaining shell implementations.
 - Common commands: `show running-config`, `show interfaces`, `enable`, `configure terminal`
 - Fake device info that looks like a real router
 
-### 3.3 Smart Fridge Shell
+### 3.3 Smart Fridge Shell ✅
 - Samsung FridgeOS boot banner
 - Inventory management commands
 - Temperature warnings
 - "WARNING: milk expires in 2 days"
-- Easter eggs
+- Per-credential shell routing via `shell` field in static credentials
 
 ### 3.4 Text Adventure
 - Zork-style dungeon crawler