VM pods: interactive exec and attach need a PTY mode in cocoon-agent

[CMGS]

Sub-issue of #657. engine/cocoon runs Execute through cocoon vm exec, which is pipe mode (stdin, stdout, stderr, exit code) because cocoon-agent has no PTY mode yet. Interactive eru-cli workload exec -t and attach on VM pods therefore work without a terminal; cocoon vm console is the PTY path meanwhile.

Blocked on cocoonstack/cocoon-agent growing a PTY mode. When it lands: pass the terminal size through vm exec, wire ExecResize to it, and make VirtualizationAttach use it. Nothing else in #657 depends on this.