pk;scfg suppress notifications is currently boolean - either "proxied messages are never @silent" (off), or "proxied messages are always @silent" (on). Add a third state to make proxied messages use the same value for SUPPRESS_NOTIFICATIONS as the trigger message.

• Maybe rename the existing states also to make the behaviour clearer? (on -> always; off -> never)
• Should this also have a "invert state of trigger message" mode? (trigger message has SUPPRESS_NOTIFICATIONS -> proxied message does not; normal trigger message -> proxied message has SUPPRESS_NOTIFICATIONS flag)

This is a followup to #596 (comment) after discussions in the support server - see https://canary.discord.com/channels/466707357099884544/1319461648251097288/1415892281948508180 for that discussion