Terminates a PTY session and optionally cleans up its buffer.

Use this tool to:
- Stop a running process (sends SIGTERM)
- Clean up an exited session to free memory
- Remove a session from the list

Usage:
- `id`: The PTY session ID (from pty_spawn or pty_list)
- `cleanup`: If true, removes the session and frees the buffer (default: false)

Behavior:
- If the session is running, it will be killed (status becomes "killed")
- If cleanup=false (default), the session remains in the list with its output buffer intact
- If cleanup=true, the session is removed entirely and the buffer is freed
- Keeping sessions without cleanup allows you to compare logs between runs

Tips:
- Use cleanup=false if you might want to read the output later
- Use cleanup=true when you're done with the session entirely
- To send Ctrl+C instead of killing, use pty_write with data="\x03"

Examples:
- Kill but keep logs: cleanup=false (or omit)
- Kill and remove: cleanup=true
