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/auth/auth.go

@@ -21,6 +21,7 @@ type credKey struct {
 type Decision struct {
 	Accepted bool
 	Reason   string // "static_credential", "threshold_reached", "remembered_credential", "rejected"
+	Shell    string // optional: route to specific shell (only set for static credentials)
 }
 
 type Authenticator struct {
@@ -50,7 +51,7 @@ func (a *Authenticator) Authenticate(ip, username, password string) Decision {
 		pMatch := subtle.ConstantTimeCompare([]byte(cred.Password), []byte(password))
 		if uMatch == 1 && pMatch == 1 {
 			a.failCounts[ip] = 0
-			return Decision{Accepted: true, Reason: "static_credential"}
+			return Decision{Accepted: true, Reason: "static_credential", Shell: cred.Shell}
 		}
 	}