Add ext-toggle extension and /ext slash command

extensions/ext-toggle.ts:
  /ext lists ~/.pi/agent/extensions/ with active/disabled markers
  and toggles individual extensions by renaming between name.ts and
  name.ts.off (pi only auto-discovers *.ts). Calls ctx.reload() so the
  change takes effect without restarting pi.

  Subdirectory-style extensions (name/index.ts) are listed read-only
  in v1 — toggling a directory cleanly is more work than the rename
  trick is worth.

install.sh:
  --uninstall now matches both *.ts and *.ts.off symlinks pointing
  into this repo, so a disabled extension is still cleaned up.

README.md / AGENTS.md:
  Document ext-toggle alongside the others; AGENTS notes the API
  surface used (registerCommand, ui.select/confirm/notify, reload)
  and the rename-not-delete design decision.

install.sh

@@ -182,13 +182,18 @@ do_uninstall() {
     echo
 
     local removed=0
-    for dest in "${EXTENSIONS_DEST}"/*.ts; do
-        [[ -e "$dest" || -L "$dest" ]] || continue
-        if link_into_repo "$dest"; then
-            rm "$dest"
-            ok "Removed $(basename "$dest")"
-            (( removed++ )) || true
-        fi
+    # Match both active (.ts) and disabled (.ts.off) symlinks — the ext-toggle
+    # extension can rename a link to .ts.off to disable it, and uninstall
+    # should still clean those up.
+    for pattern in "*.ts" "*.ts.off"; do
+        for dest in "${EXTENSIONS_DEST}"/$pattern; do
+            [[ -e "$dest" || -L "$dest" ]] || continue
+            if link_into_repo "$dest"; then
+                rm "$dest"
+                ok "Removed $(basename "$dest")"
+                (( removed++ )) || true
+            fi
+        done
     done
 
     if [[ $removed -eq 0 ]]; then