Group @ of a friend's Bot: send failures and denies are silent (#73's pause message only shows in the private chat) #81

Follow-up from #80 (same live incident, same send path — but a separate fix).

What happens

sendUserMentions (core/src/host/hooks.js:546) is the only path that delivers a user's group @ of a friend's Bot, and every failure mode is invisible in the UI:

• Transport errors are caught and only logged: core/src/host/hooks.js:573-576 (warn('flybot: sending to %s failed: %s', …)). social: map Bot Social conversation pauses to an actionable error (#72) #73's actionable pause message — "Bot Social paused this conversation (…). Resending cannot help: approve pending messages or extend the conversation in the Bot Social console, or remove and re-add this friend" — is mapped in the bot-social transport (plugins/social/src/social.js:359-368) and thrown, but the group path swallows it. While a conversation is paused_limit (the Friend chat: Bot Social conversation pause (turn budget) surfaces only as a bare "Not sent" card, with no state or recovery path #72 scenario: the 20-turn budget burned by held messages), every group @ fails with no trace; the bubble renders as a normal sent message.
• Denies come back as { action: 'deny', reason } — unknown handle (plugins/social/src/social.js:342) or a sender the friendship doesn't cover (plugins/social/src/social.js:343-344, e.g. "Chief is not friends with @danyang.home: that friendship is with @flybot-manager.hsliu.home only"). core/src/host/hooks.js:591 only uses them to decide whether to cancel the local turn; the reason is never shown.
• Holds are fine — they produce the local approval card.

Repro

1. A friend conversation is paused (paused_limit; see Friend chat: Bot Social conversation pause (turn budget) surfaces only as a bare "Not sent" card, with no state or recovery path #72).
2. In a group chat, @ the friend's Bot and send.
3. The message renders as sent. No notice, no badge. Server-side nothing left (409 conversation_paused). The only record is the host log: flybot: sending to @… failed: Bot Social paused this conversation ….

This happened in practice today: after the old conversation paused, group mentions kept "working" visually while nothing left.

Suggestion

Surface the per-mention outcome in the chat the user typed in — a system/error notice for deny and thrown errors, reusing #73's mapped text so the recovery guidance (extend in the Bot Social console / remove and re-add) is in front of the user. Holds already have their card.

Related: #72 (closed by #73 — private-chat side), #80 (reply routing in groups).