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/shell/shell.go

@@ -2,6 +2,7 @@ package shell
 
 import (
 	"context"
+	"fmt"
 	"io"
 
 	"git.t-juice.club/torjus/oubliette/internal/storage"
@@ -31,3 +32,59 @@ type ShellCommonConfig struct {
 	Banner   string
 	FakeUser string // override username in prompt; empty = use authenticated user
 }
+
+// ReadLine reads a line of input byte-by-byte, handling backspace, Ctrl+C, and Ctrl+D.
+func ReadLine(ctx context.Context, rw io.ReadWriter) (string, error) {
+	var buf []byte
+	b := make([]byte, 1)
+
+	for {
+		select {
+		case <-ctx.Done():
+			return "", ctx.Err()
+		default:
+		}
+
+		n, err := rw.Read(b)
+		if err != nil {
+			return "", err
+		}
+		if n == 0 {
+			continue
+		}
+
+		ch := b[0]
+		switch {
+		case ch == '\r' || ch == '\n':
+			fmt.Fprint(rw, "\r\n")
+			return string(buf), nil
+
+		case ch == 4: // Ctrl+D
+			if len(buf) == 0 {
+				return "", io.EOF
+			}
+
+		case ch == 3: // Ctrl+C
+			fmt.Fprint(rw, "^C\r\n")
+			return "", nil
+
+		case ch == 127 || ch == 8: // DEL or Backspace
+			if len(buf) > 0 {
+				buf = buf[:len(buf)-1]
+				fmt.Fprint(rw, "\b \b")
+			}
+
+		case ch == 27: // ESC - start of escape sequence
+			// Read and discard the rest of the escape sequence.
+			// Most are 3 bytes: ESC [ X (arrow keys, etc.)
+			next := make([]byte, 1)
+			if n, _ := rw.Read(next); n > 0 && next[0] == '[' {
+				rw.Read(next) // read the final byte
+			}
+
+		case ch >= 32 && ch < 127: // printable ASCII
+			buf = append(buf, ch)
+			rw.Write([]byte{ch})
+		}
+	}
+}