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/server/server_test.go

@@ -108,7 +108,7 @@ func TestIntegrationSSHConnect(t *testing.T) {
 			AcceptAfter:       2,
 			CredentialTTLDuration: time.Hour,
 			StaticCredentials: []config.Credential{
-				{Username: "root", Password: "toor"},
+				{Username: "root", Password: "toor", Shell: "bash"},
 			},
 		},
 		Shell: config.ShellConfig{