The on_processing_start hook fired a reaction emoji (👀) on every
inbound Signal message before run.py's _is_user_authorized check.
This meant contacts not in SIGNAL_ALLOWED_USERS would see the bot
react to their messages even though Hermes silently dropped them —
leaking the presence of the bot and causing confusing UX.
Two changes to gateway/platforms/signal.py:
1. Read SIGNAL_ALLOWED_USERS into self.dm_allow_from in __init__
(mirrors the group_allow_from pattern already in place).
2. Add _reactions_enabled(event) — two-gate check:
- SIGNAL_REACTIONS=false/0/no disables reactions globally
- If SIGNAL_ALLOWED_USERS is set, only react to senders in
the allowlist (skips unauthorized contacts)
Both on_processing_start and on_processing_complete now call this
guard before sending any reaction.
Telegram already has an equivalent _reactions_enabled() guard
(controlled by TELEGRAM_REACTIONS). This brings Signal to parity.
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