Shadow mode for safe rollout

[hoalongnatsu]

Phase: phase-1
Estimate: S
Section: Deliver incidents through existing channels

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Goal

Before going live, operators can run in "shadow mode" where the agent logs what it would have reported but doesn't actually send alerts. This makes it safe to validate accuracy before enabling real notifications.

Acceptance criteria

Out of scope

Notes

• See phase-1-logs.md and architecture.md for implementation details.