Slash commands (/exit, /clear) broken on iOS mobile app due to system-reminder timestamp injection

Description

Slash commands like /exit and /clear stopped working when sent from the iOS Claude Code app (remote/mobile session). This regression was introduced when the harness started injecting a <system-reminder>Message sent at ...</system-reminder> timestamp prefix into messages sent from mobile clients.

Root Cause

The slash command parser detects commands by looking for / at the start of the message. However, messages sent from iOS now arrive with the system-reminder prepended:

<system-reminder>Message sent at Mon 2026-06-15 16:39:18 UTC.</system-reminder>
/exit

Since the message no longer starts with /, the command is not recognized and is treated as plain text instead.

Steps to Reproduce

1. Open a Claude Code session in remote-control mode (e.g. spawned via a launcher on a remote host)
2. Connect to it from the iOS Claude Code app
3. Type /exit or /clear and send
4. Command is not executed — treated as a plain text message

Expected Behavior

/exit and /clear (and any other slash commands) should work from the iOS app, regardless of whether the harness prepends a system-reminder timestamp.

Actual Behavior

Slash commands are silently ignored (treated as plain text).

Suggested Fix

The harness should parse and dispatch slash commands before injecting the system-reminder, or should strip/ignore the system-reminder prefix when checking for slash commands at the start of user input.

Environment

• Claude Code version: 2.1.177
• Client: iOS Claude Code app (remote session)
• Host: Linux (Ubuntu)