ext-toggle: refuse to disable ssh-controlmaster during --ssh session

Disabling ssh-controlmaster mid --ssh session would tear down the
ControlMaster (if we own it) and silently redirect read/write/edit/bash
back to the local filesystem while the system prompt still claims we're
on the remote. Now blocked with an explanatory dialog.

Implementation: a DISABLE_GUARDS map keyed by bare extension name lets
specific extensions register a refusal predicate. ssh-controlmaster's
guard checks process.argv for --ssh and refuses if present. Easy to
extend with similar foot-guns later.

README.md

@@ -177,6 +177,7 @@ Registers `/ext` — a slash command that lists extensions in `~/.pi/agent/exten
 - Subdirectory-style extensions (`name/index.ts`) are listed read-only — v1 doesn't toggle them. Move the directory aside manually if needed.
 - `install.sh --uninstall` cleans up both `.ts` and `.ts.off` symlinks pointing into this repo, so a disabled extension won't be left behind.
 - Re-running `./install.sh` respects a prior `/ext` disable: if `<name>.ts.off` already exists, the installer leaves it alone instead of silently re-enabling.
+- `ssh-controlmaster` cannot be disabled via `/ext` while pi was launched with `--ssh` — disabling mid-session would silently revert tool calls to the local filesystem. Exit pi and relaunch without `--ssh` instead.
 
 ## Adding a new extension