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Adds ObservationStore (core:context) keyed on (repoRoot, path), an in-memory default and a durable SqliteObservationStore (infrastructure:persistence), and wires it into the two SessionOrchestratorArtifacts call sites that re-derive a SourceDescriptor from CAS bytes on every call — a hit on matching content hash skips both the CAS read and the regex extraction. First slice of docs/plans/2026-07-21-acr-knowledge-accretion.md (build order: observations before fixes/plan-shapes). Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01HMbPmZZjcXhR2crU82zZ8S