feat(acr): ACR Store 1 — content-hash observation cache for repo-file descriptors (#305)

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

core/context/build.gradle

@@ -8,6 +8,8 @@ dependencies {
     implementation(project(":core:events"))
     implementation(project(":core:artifacts"))
     implementation(project(":core:sessions"))
+    testImplementation "org.jetbrains.kotlin:kotlin-test"
+    testImplementation "org.junit.jupiter:junit-jupiter"
 }
 
 tasks.named("koverVerify").configure { enabled = false }