[ARI-36] Validate AC-05 — Confidence band shown in 100% of notifications

[bjridicodes]

Linear: ARI-36

Type: 🔵 User Story | Sprint: 7 | Points: 2 | AC: AC-05

As an on-call engineer, I want every single ARIA notification to show a confidence band so that I never receive a finding without knowing how much to trust it.

Acceptance Criteria

Given 10 test incidents are processed
When each Slack notification is inspected
Then 10 of 10 contain a visible confidence band (high / medium / low) — AC-05 requires 100%

Given any notification with LOW confidence
When it is inspected
Then it contains explicit language indicating uncertainty (e.g. "low confidence — treat as unverified")
And it does NOT contain unqualified statements like "root cause is X"

Given 1 incident where Agent 3 fails entirely
When the notification is sent
Then it still shows "confidence: unknown — classification failed" rather than omitting the field