feat(alerts): signal-type-aware cooldown (Issue vs MonitorMissed) #366

Summary

Alert cooldown is per-rule across signal types: an Issue alert and a MonitorMissed alert sharing one rule window can suppress each other. Found in #364 security review (parity with old logic, not a regression).

Why

A missed-monitor alert being silenced by an unrelated recent issue alert (or reverse) is surprising. Signal-type-aware cooldown windows give correct urgency for each type.

Acceptance criteria

• Cooldown keyed by (rule, signal-type) — Issue and MonitorMissed tracked independently
• Tests prove cross-type suppression no longer occurs