feat(884): persistent perm_rx lock-holder for Matrix bot

Before: handle_message.rs acquired services.perm_rx only while processing
one chat message and dropped it on chat_fut completion. The moment the
bot wasn't actively responding, prompt_permission auto-denied any spawned
coder bash call as "no interactive session" — making unattended coder
work impossible.

Now: a permission_listener task is spawned at bot startup and holds
perm_rx for the bot's lifetime. Permission requests are forwarded to
the first configured Matrix room, replies resolved by the existing
on_room_message handler via pending_perm_replies. Per-message acquire is
gone from handle_message.rs (chat_fut just awaits cleanly).

- New module: chat/transport/matrix/bot/permission_listener.rs.
- Wired into run_bot before BotContext construction; bot_sent_event_ids
  is hoisted out so the listener and the rest of the bot share it.
- handle_message.rs no longer touches perm_rx.
- diagnostics/permission.rs comment updated to reflect the new reality.
- Regression test asserts the listener forwards a PermissionForward to
  the target room and records the pending reply key — exactly the path
  that was broken when no chat_fut was in flight.

Discord/Slack/WhatsApp transports still acquire perm_rx per message
(commands.rs:368 / commands/llm.rs:83 / commands/llm.rs:82). They are
not the active transport in this deployment so their per-message acquire
remains dormant; the same listener pattern should be applied to them as
follow-up work in 884 phase 2.

server/src/chat/transport/matrix/bot/mod.rs

@@ -9,6 +9,9 @@ pub mod history;
 pub mod mentions;
 /// Message handlers — processes incoming Matrix room messages.
 pub mod messages;
+/// Permission listener — holds perm_rx for the bot's lifetime and forwards
+/// permission requests to the configured Matrix room.
+pub mod permission_listener;
 /// Bot run loop — the main async task that drives the Matrix sync loop.
 pub mod run;
 /// Device verification — handles Matrix cross-signing and emoji verification flows.