Add LXC shell via ssh and pct enter

Press t on a running container to suspend the TUI and run
`ssh -t root@<node> pct enter <vmid>`. cv4pve-cli has no shell or exec
for LXC, so this goes around it and needs key-based root SSH to the
PVE node.

- VMs, stopped containers and busy guests get a warning instead.
- Only exit code 255 (ssh failed) is reported as an error. Other codes
  are the last status of the container's shell.
- OSError is caught inside suspend(). Textual's suspend() has no
  try/finally, so an exception escaping it would leave the app
  suspended.
- The guest table reloads when the shell returns.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -20,10 +20,13 @@ uv run cv4pve-tui
 | `x` | stop - hard power-off (confirm) |
 | `r` | reboot (confirm) |
 | `e` | edit cores / memory / grow a disk (disk growth asks again; it cannot be undone) |
+| `t` | shell in a running LXC container (`ssh -t root@<node> pct enter <vmid>`) |
 | `g` | refresh |
 | `c` | switch cv4pve-cli context (cluster) |
 | `q` | quit |
 
+The shell key needs key-based root SSH to the PVE node (LXC only).
+
 Switching context runs `cv4pve-cli config use`, which changes cv4pve-cli's global current
 context: other shells and scripts follow, and it stays after the app exits.