Reminder fires per-scope, multiplies noise across rooms

[joelteply]

Symptom (continuum-b741 review, 2026-04-27)

Reminder fires per-scope independently (I just got two 5-min reminders, one in #cambriantech and one in #general, ~10s apart from this tab's perspective). For a user in 5 rooms that's a 5x noise multiplier. Unsure if by-design.

Each subscribed scope (primary + each sidecar) runs its own silence-reminder timer. The user sees one reminder per scope, even though "they" haven't sent across any of them.

Resolution call

Three options:

1. Status quo (by design): each room is independently active; an idle agent in #cambriantech might still be active in #general and vice versa. Reminder accuracy at the cost of noise.
2. Tab-level reminder: track silence at the tab/Monitor level (across all subscribed scopes). One reminder if the user is silent across ALL their rooms. Less noisy; loses room-specific signal.
3. Reminder coalescing: fire a single composite reminder when multiple scopes hit the threshold within N seconds. "Idle in #cambriantech and #general — silence reminder."

Option 3 is the natural compromise. Default-on; `airc reminder per-scope` flag for users who genuinely want per-room signal.

Out of scope for #144

Design call for next iteration. Not a regression — pre-existing behavior.