Add interactive choice-picker menus for finite-choice slash commands

Slash commands with a finite set of options (/reasoning, /fast, ...) now
render a tappable card with buttons (2 per row, ✓ on the current value)
instead of a plain text status card. The mechanism is generic: any command
that calls the adapter's send_choice_picker() gets a picker automatically.

Wire protocol (docs/04, frames.schema.json):
- picker.choice (server→app): {picker_id, title, choices[]}
- picker.select (app→server): {picker_id, value}
- pickers capability flag now True in server_caps

gateway-plugin:
- protocol.py: picker.choice/picker.select frame types + picker_choice()
- dispatch.py: route picker.select → adapter.on_picker_select
- adapter.py: send_choice_picker() (fails cleanly with no live device so
  hermes falls back to text), on_picker_select(), in-memory pending pickers
  (gateway restart expires them; stale select is a no-op), pickers=True

app (KMP):
- Protocol.kt: PickerChoice/PickerChoicePayload + pickerSelectFrame()
- ChatStore.kt: PickerItem + onPickerChoice (idempotent) + resolvePicker
  (optimistic, one-shot)
- ChatDb.kt: persist PickerItem in the messages table (polymorphic decode)
- IrisController.kt: picker.choice routing + selectPicker() action
- ChatScreen.kt: PickerCard composable (locks after selection)

Tests:
- python: 3 picker tests (roundtrip, no-device fallback, stale-select noop)
- kotlin: ChatStorePickerTest (add/idempotent/resolve/one-shot/noop/serialize)
- fixture fix: clear leaked IRIS_HTTP_PORT/IRIS_WS_HOST env so the adapter
  binds the ephemeral port (a prior test's interactive_setup() polluted the
  process env, colliding with a live gateway on 8791)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

gateway-plugin/dispatch.py

@@ -57,6 +57,8 @@ async def dispatch_frame(adapter: Any, frame: protocol.Frame, device_id: str) ->
         await adapter.on_message_delete(frame, device_id)
     elif frame.type == protocol.TYPE_FCM_REGISTER:
         await adapter.on_fcm_register(frame, device_id)
+    elif frame.type == protocol.TYPE_PICKER_SELECT:
+        await adapter.on_picker_select(frame, device_id)
     # Unknown types are ignored (forward-compat).