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.

AGENTS.md

@@ -21,6 +21,7 @@ extensions/
   confirm-destructive.ts  # Confirm before dangerous bash commands and session actions
   git-checkpoint.ts       # Git stash checkpoint per turn, restorable on /fork
   notify.ts               # Native terminal notification when agent finishes
+  ext-toggle.ts           # /ext slash command — list & toggle extensions at runtime
 install.sh                # Idempotent installer — symlinks extensions/ into ~/.pi/agent/extensions/
 package.json              # pi package manifest — enables `pi install /path` as an alternative
 README.md                 # User-facing docs.
@@ -159,6 +160,43 @@ Terminal detection order: `KITTY_WINDOW_ID` → OSC 99 (Kitty) →
 
 Notification text: `Pi — Done (Ns)` where N is the rounded elapsed seconds.
 
+### `ext-toggle.ts`
+
+Registers `/ext` slash command. Lists files in `~/.pi/agent/extensions/`,
+shows `●` (active) / `○` (disabled) plus dir/symlink hints, and lets the
+user toggle individual extensions by renaming them between `name.ts` and
+`name.ts.off`. Calls `ctx.reload()` after a toggle so the change takes
+effect without restarting pi.
+
+**Key design decisions:**
+
+- **Rename, not delete.** Disabling a built-in produces a `name.ts.off`
+  symlink/file that's invisible to pi's `*.ts` discovery glob but trivially
+  reversible. No state stored elsewhere.
+- **Symlink-friendly.** `fs.renameSync` renames the symlink itself; the repo
+  target is untouched. Toggling an extension installed by this repo is
+  reversible without re-running `install.sh`.
+- **Subdir extensions are read-only in v1.** `name/index.ts` shapes show up in
+  the listing with a `[dir]` tag but cannot be toggled — the cleanest disable
+  for a directory would need a hidden-prefix or move-aside dance that adds
+  more failure modes than it's worth for now.
+- **`install.sh --uninstall` matches both `*.ts` and `*.ts.off`.** Means a
+  disabled extension is still cleaned up on uninstall, regardless of which
+  state it was left in.
+- **Listing is recomputed on every `/ext` invocation.** No cache, no event
+  subscription — cheap enough for the tens of files this directory will ever
+  contain.
+
+**API used:**
+
+- `pi.registerCommand(name, { description, handler })` — registers `/ext`.
+- `ctx.ui.select(title, items)` — picker; returns selected string or `undefined`.
+- `ctx.ui.confirm(title, message)` — yes/no dialog returning `boolean`.
+- `ctx.ui.notify(message, level)` — transient toast.
+- `ctx.reload()` — reloads extensions/skills/prompts/themes; same as `/reload`.
+
+No flags, no `agent_*` event handlers — fully passive until `/ext` is invoked.
+
 No framework. Manual:
 
 ```bash