ext-toggle: self-guard against disabling /ext

Disabling ext-toggle through its own /ext UI would rename the file to
ext-toggle.ts.off, which pi's auto-discovery skips, so the next /reload
silently drops the /ext command itself. The .ts.off rename also
persists across pi restarts and \u2014 in containerized setups
(opencode-devbox) where ~/.pi is mounted on the devbox-pi-config named
volume \u2014 across container recreate, so even nuking the container
doesn't recover the surface.

Add ext-toggle to the existing DISABLE_GUARDS map so the toggle is
refused at stage-time with an explanation pointing at the manual
recovery path:

    mv ~/.pi/agent/extensions/ext-toggle.ts.off \
       ~/.pi/agent/extensions/ext-toggle.ts

Same shape as the existing ssh-controlmaster guard. Sibling slash
commands (/mcp from mcp-loader) don't need this guard because
settings.json is their source of truth and remains editable by hand
even if the loader is disabled \u2014 only ext-toggle's own UI is the
single point of management.

extensions/ext-toggle.ts

@@ -58,6 +58,18 @@ const DISABLE_GUARDS: Record<string, () => string | null> = {
     if (!hasSshFlag) return null;
     return "Active --ssh session — disabling would silently revert read/write/edit/bash to local. Exit pi and relaunch without --ssh.";
   },
+  // ext-toggle owns the /ext slash command. Disabling it removes the
+  // only TUI surface for re-enabling itself: pi auto-discovers `*.ts`
+  // only, so once renamed to `.ts.off` the file is invisible to pi and
+  // the next /reload silently drops the command. The disabled state
+  // also persists — in containerized setups (opencode-devbox) the
+  // ~/.pi volume keeps the `.ts.off` rename across container recreate,
+  // so even nuking the container doesn't recover the surface. Recovery
+  // path is manual: shell into the container (or open a host shell),
+  // run `mv ~/.pi/agent/extensions/ext-toggle.ts.off \
+  // ~/.pi/agent/extensions/ext-toggle.ts`, then /reload.
+  "ext-toggle": () =>
+    "Disabling ext-toggle would remove the /ext command itself — only manual `mv ~/.pi/agent/extensions/ext-toggle.ts.off ~/.pi/agent/extensions/ext-toggle.ts` recovers it. Refusing.",
 };
 
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