feat(terminal): add clear and restart commands

[tarik02]

Problem

The terminal command group covers creation, attachment, I/O, waiting, and destruction, but omits the server's clear and restart operations.

Scope

Add:

• terminal clear
• terminal restart

Both commands should use the existing thread and terminal ID resolution rules.

Acceptance criteria

• terminal clear clears the target terminal's retained output through terminal.clear.
• terminal restart restarts the target terminal through terminal.restart.
• Human and JSON output identify the affected terminal and returned state.
• Missing, closed, and unauthorized terminals use the existing structured terminal error path.