fix(workflow,kernel): split plan stage 'produces' (slot name) from 'kind' (artifact kind)

ExecutionPlanCompiler treated the plan's single 'produces' string as a
registry kind id, while the architect prompt described it as a unique
artifact id referenced by needs/edges — the model followed the prompt,
invented descriptive ids (files_created), and every freestyle plan
failed to compile. Collapsing both onto one string is also unsound:
two stages producing the same kind would collide on slot name and make
artifact_validated edge conditions ambiguous.

Plan stages now carry an optional 'kind' selecting the registered kind
(mirroring TOML's produces = { name, kind }); 'produces' stays the
unique slot name. Missing 'kind' falls back to the old produces-as-kind
reading so existing plans still compile. Vocabulary entry, architect
prompt, and execution_plan schemas updated to match.

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

@@ -29,8 +29,11 @@ class ExecutionPlanCompiler(
         if (plan.stages.isEmpty()) throw WorkflowValidationException("execution_plan has no stages")
 
         val stageMap = plan.stages.associate { s ->
-            val kind = registry.get(s.produces)
-                ?: throw WorkflowValidationException("Unknown artifact kind '${s.produces}' in stage '${s.id}'")
+            // `produces` is the unique slot name referenced by needs/edges; `kind` selects the
+            // registered artifact kind. Plans predating the split used one string for both.
+            val kindId = s.kind ?: s.produces
+            val kind = registry.get(kindId)
+                ?: throw WorkflowValidationException("Unknown artifact kind '$kindId' in stage '${s.id}'")
             StageId(s.id) to StageConfig(
                 produces = listOf(TypedArtifactSlot(name = ArtifactId(s.produces), kind = kind)),
                 needs = s.needs.map { ArtifactId(it) }.toSet(),

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

@@ -12,6 +12,7 @@ data class PlanStage(
     val id: String = "",
     val prompt: String = "",
     val produces: String = "",
+    val kind: String? = null,
     val needs: List<String> = emptyList(),
     val tools: List<String> = emptyList(),
 )

infrastructure/workflow/src/test/kotlin/com/correx/infrastructure/workflow/ExecutionPlanCompilerTest.kt

@@ -87,6 +87,48 @@ class ExecutionPlanCompilerTest {
         assertThrows<WorkflowValidationException> { compiler.compile(bad, "bad-workflow") }
     }
 
+    @Test
+    fun `kind field selects the registered kind while produces names the slot`() {
+        val plan = """
+            {
+              "goal": "create files",
+              "stages": [
+                {
+                  "id": "create_files",
+                  "prompt": "Create the files",
+                  "produces": "files_created",
+                  "kind": "file_written",
+                  "needs": [],
+                  "tools": ["file_write"]
+                }
+              ],
+              "edges": [
+                { "from": "create_files", "to": "done", "condition": { "type": "always_true" } }
+              ]
+            }
+        """.trimIndent()
+        val graph = compiler.compile(plan, "kind-workflow")
+        val slot = graph.stages.values.single().produces.single()
+        assertEquals("files_created", slot.name.value)
+        assertEquals("file_written", slot.kind.id)
+    }
+
+    @Test
+    fun `unregistered kind field throws WorkflowValidationException`() {
+        val plan = """
+            {
+              "goal": "x",
+              "stages": [
+                { "id": "s1", "prompt": "p", "produces": "out", "kind": "made_up_kind" }
+              ],
+              "edges": [
+                { "from": "s1", "to": "done", "condition": { "type": "always_true" } }
+              ]
+            }
+        """.trimIndent()
+        assertThrows<WorkflowValidationException> { compiler.compile(plan, "bad-workflow") }
+    }
+
     @Test
     fun `empty stages throws WorkflowValidationException`() {
         val bad = """{ "goal": "x", "stages": [], "edges": [] }"""