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>

oubliette.toml.example

@@ -18,6 +18,12 @@ password = "toor"
 username = "admin"
 password = "admin"
 
+# Route specific credentials to a named shell (optional).
+# [[auth.static_credentials]]
+# username = "samsung"
+# password = "fridge"
+# shell = "fridge"
+
 [storage]
 db_path = "oubliette.db"
 retention_days = 90