Superseded by !11529/!11530/!11531 — Zoekt observability: dashboard, alert rules and runbooks for the ORR

🤖 AI-authored change.

Superseded — do not review this merge request. It was declined as too large to review and has been split into three smaller ones, which carry the whole diff between them.

Order	Merge request	Contents
1	!11529 (merged)	Dashboard and its two reducer guards
2	!11530 (merged)	8 alert rules, jsonnet + generated YAML + promtool tests
3	!11531 (merged)	9 docs/zoekt/ runbook pages, prose only

Every file blob of this branch is byte-identical at the top of that stack, so nothing was dropped in the split. Review order is smallest first; each targets the one above it.

The original write-up this description carried — the threshold sourcing table, the for: reconciliation, the drain-ETA rewrite, the mutation-testing results and the complete gap list — is unchanged at $6053783

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