Add decision record for DiscoveryPolicy query/signal/ranking pipeline

[Copilot]

Captures the decided redesign of DiscoveryPolicy discovery — replacing the single-axis spec.sources[] model with an explicit queries → signals → ranking pipeline — as a decision record so implementation can start when prioritized. Docs-only; no code or API changes yet.

What's added

• docs/decisions/13-discovery-signals-ranking.md — a detailed PDR covering the full target design.

Contents

• Decision & rationale — breaking replacement of sources[] (no v1alpha1 compat guarantee, no installed resources to migrate); registry tag discovery dropped for v1; Issue 1 (CRD types) lands first as the foundation.
• Target CRD shape — retained fields (syncInterval, maxImages, imageFilter) plus the new queries/signals/ranking stages.
• Per-stage field reference — Prometheus + Loki query types (incl. kubernetesEvents parser), four signal types (aggregate, timeWeightedAggregate, windowAggregate, eventPullTime), three ranking strategies (signal, weightedSum, modelExposure) with exact score formulas.
• Strategy catalog — 9 discovery strategies mapped to required signals/queries, with a recommended delivery order.
• Status & observability — explainable per-image breakdown (raw/normalized values, ranking contributions), query/signal result reporting, print-column changes.
• Validation plan & package placement — aligned to existing conventions (pure-function signal/ranking packages, envtest, Chainsaw e2e).
• Implementation split — 11 sequential issues from CRD types through docs.
• Open decisions — missing-signal behavior, pull-time statistic/source, decimal field type, status object size, registry removal — each with a recommendation.

Notes

• Supersedes the discovery section of 12-naming-structure-proposals.md; that file and 10-policy-redesign-proposals.md still describe the old sources[] shape and are left untouched pending a follow-up pointer.