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/bash/bash.go

@@ -55,7 +55,7 @@ func (b *BashShell) Handle(ctx context.Context, sess *shell.SessionContext, rw i
 			return nil
 		}
 
-		line, err := readLine(ctx, rw)
+		line, err := shell.ReadLine(ctx, rw)
 		if errors.Is(err, io.EOF) {
 			fmt.Fprint(rw, "logout\r\n")
 			return nil
@@ -103,58 +103,3 @@ func formatPrompt(state *shellState) string {
 	return fmt.Sprintf("%s@%s:%s# ", state.username, state.hostname, cwd)
 }
 
-// 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})
-		}
-	}
-}