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.

README.md

@@ -161,9 +161,28 @@ pi --notify-min-secs 15   # only notify for tasks over 15 seconds
 pi --notify-min-secs 0    # notify on every agent completion
 ```
 
+### `ext-toggle.ts`
+
+Registers `/ext` — a slash command that lists extensions in `~/.pi/agent/extensions/` and toggles individual ones on/off without leaving the TUI.
+
+**How it works:** pi auto-discovers `*.ts` only. Toggling renames a file (or symlink) between `name.ts` and `name.ts.off`, so a disabled extension is invisible to the loader. After a toggle, the extension calls `ctx.reload()` so the change takes effect immediately — no restart needed.
+
+**Usage:**
+
+```
+/ext                # opens a picker; ● = active, ○ = disabled
+```
+
+**Notes:**
+- 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.
+
+## Adding a new extension
+
 1. Drop a `.ts` file into `extensions/`
 2. Re-run `./install.sh` — it picks up the new file and symlinks it
 3. In a running pi session, `/reload` is enough; no restart needed
+4. (or, with `ext-toggle` installed: `/ext` to disable noisy ones at runtime)
 
 Each extension is a TypeScript module loaded by [jiti](https://github.com/unjs/jiti) — no compilation step. See the [pi extensions docs](https://github.com/mariozechner/pi-coding-agent/blob/main/docs/extensions.md) and the [built-in examples](https://github.com/mariozechner/pi-coding-agent/tree/main/examples/extensions) for the API surface.