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>

src/cv4pve_tui/app.py

@@ -1,7 +1,9 @@
 """Textual app for managing Proxmox guests through cv4pve-cli."""
 
+import subprocess
+
 from textual import work
-from textual.app import App, ComposeResult
+from textual.app import App, ComposeResult, SuspendNotSupported
 from textual.containers import Horizontal, Vertical
 from textual.screen import ModalScreen
 from textual.widgets import (
@@ -132,6 +134,7 @@ class GuestApp(App):
         ("x", "act('stop')", "Stop"),
         ("r", "act('reboot')", "Reboot"),
         ("e", "edit", "Edit"),
+        ("t", "shell", "Shell"),
         ("g", "reload", "Refresh"),
         ("c", "context", "Context"),
         ("q", "quit", "Quit"),
@@ -336,6 +339,39 @@ class GuestApp(App):
             return
         self._open_edit(guest)
 
+    def action_shell(self) -> None:
+        guest = self._selected()
+        if guest is None:
+            return
+        who = f"{guest['name']} ({guest['vmid']})"
+        if guest["type"] != "lxc":
+            self.notify(f"{who} is not a container", severity="warning")
+            return
+        if guest["status"] != "running":
+            self.notify(f"{who} is not running", severity="warning")
+            return
+        if guest["vmid"] in self.busy:
+            self.notify(f"{who} is busy", severity="warning")
+            return
+        cmd = ["ssh", "-t", f"root@{guest['node']}", "pct", "enter", str(guest["vmid"])]
+        error = None
+        try:
+            with self.suspend():
+                try:  # inside: Textual's suspend() skips the resume if its body raises
+                    code = subprocess.run(cmd, check=False).returncode
+                except OSError as exc:
+                    error = _message(exc)
+        except SuspendNotSupported as exc:
+            error = _message(exc)
+        else:
+            if (
+                error is None and code == 255
+            ):  # other codes are the shell's own last status
+                error = "ssh failed (exit 255)"
+        if error:
+            self.notify(f"Shell in {who} failed: {error}", severity="error")
+        self.action_reload()
+
     @work(thread=True)
     def _open_edit(self, guest: dict) -> None:
         try: