[BUG] Remote Control: "Message sent at ..." timestamp prefix breaks slash command recognition (web + mobile)

Summary

Messages sent via Remote Control (both the mobile app and the claude.ai/code web UI) get a <system-reminder>Message sent at <timestamp>.</system-reminder> block prepended before the user's text. Because slash commands are only recognized when the message begins with / at position 0, this prefix pushes the command out of position and it is treated as a plain-text prompt instead of executing. Slash commands (/clear, /context, …) are broken from all Remote Control clients.

Reproduction

1. Attach to a running session via Remote Control (web UI or mobile).
2. Send /context (or /clear).
3. The model receives:

``
<system-reminder>Message sent at Mon 2026-06-15 20:13:33 UTC.</system-reminder>
/context
``

1. The command does not fire — it arrives as a normal prompt.

Expected: Slash commands are recognized and executed regardless of prepended metadata (timestamp should not interfere with command parsing).

Actual: /-commands are treated as plain text and never execute from Remote Control.

Environment

• Claude Code: 2.1.178 (latest; newer than the 2.1.176 Remote Control fixes — not resolved by updating)
• Host OS: Ubuntu 24.04 LTS, X11
• Reproduced from: both web UI (claude.ai/code) and mobile app → points to the shared Remote Control layer, not a client-specific bug

Notes

• No settings.json option to disable the timestamp injection appears to exist.
• Likely a recent regression: the Message sent at … timestamp seems intentionally added (async timing context) but breaks command parsing as a side effect.
• Possibly related: #29156, #28764, #30674.