Add slash commands to chat TUI (/exit, /quit, /help, /clear)

[pavelanni]

Problem

The chat TUI only supports Ctrl+C to exit. Users instinctively type
/exit or /quit to leave, which gets sent as a message to the agent.

Proposed commands

Command	Action
/exit, /quit	Exit the chat (same as Ctrl+C)
/help	Show available commands and key bindings
/clear	Clear the chat display (keep session)
/session	Show current session ID

Implementation

Intercept input starting with / in the Update method before
sending to the agent. Unknown / commands should show an error
rather than being sent as messages.