feat(cas): content-addressed artifact store (steps 1–8)

New core:artifacts-store interface + infrastructure/artifacts-cas
implementation: segment files + SQLite index, Blake3 hashing,
group-commit fsync via flushBefore, recovery tail-scan, manual
compactor, and oldest-first disk-cap evictor.

Inference events now carry promptArtifactId / responseArtifactId;
orchestrators put bytes before emitting. SqliteEventStore wraps
its txn in artifactStore.flushBefore so segment data is fsynced
before the event commit, making the crash window non-corrupting
(TailScanner re-indexes orphan tail records on reopen).

Compactor and evictor are mutually exclusive via maintenanceMutex.
Step 9 (cloud sync) deferred to a later epic.

See docs/reviews/2026-05-18-cas-steps-1-8-review.md for the final
review.

infrastructure/workflow/src/main/kotlin/com/correx/infrastructure/workflow/TomlWorkflowLoader.kt

@@ -24,6 +24,7 @@ private data class WorkflowFile(
 private data class StageSection(
     val id: String = "",
     val prompt: String? = null,
+    val systemPrompt: String? = null,
     val produces: List<String> = emptyList(),
     val needs: List<String> = emptyList(),
     @JsonProperty("allowed_tools") val allowedTools: List<String> = emptyList(),
@@ -66,7 +67,10 @@ class TomlWorkflowLoader : WorkflowLoader {
                 allowedTools = s.allowedTools.toSet(),
                 tokenBudget = s.tokenBudget,
                 maxRetries = s.maxRetries,
-                metadata = buildMap { s.prompt?.let { put("prompt", it) } },
+                metadata = buildMap {
+                    s.prompt?.let { put("prompt", it) }
+                        ?: s.systemPrompt?.let { put("systemPrompt", it) }
+                },
             )
         }