Replaces the permanent "OK" receipt reaction with a 3-phase visual
lifecycle:
- Typing animation appears when the agent starts processing.
- Cleared when processing succeeds — the reply message is the signal.
- Replaced with CrossMark when processing fails.
- Cleared when processing is cancelled or interrupted.
When Feishu rejects the reaction-delete call, we keep the Typing in
place and skip adding CrossMark. Showing both at once would leave the
user seeing both "still working" and "done/failed" simultaneously,
which is worse than a stuck Typing.
A FEISHU_REACTIONS env var (default on) disables the whole lifecycle.
User-added reactions with the same emoji still route through to the
agent; only bot-origin reactions are filtered to break the feedback
loop.
Change-Id: I527081da31f0f9d59b451f45de59df4ddab522ba
520edd3499