feat(workflow): execution_plan artifact kind + schema + architect prompt

Add docs/schemas/execution_plan.json (goal/stages/edges), the
architect_freestyle prompt that instructs emitting the execution pipeline as
JSON, and register the execution_plan llm-emitted kind in both
artifacts.config.toml and sample-config.toml. Validated via
ExecutionPlanSchemaValidationTest (minimal plan passes; missing stages rejected).

docs/schemas/execution_plan.json

@@ -0,0 +1,45 @@
+{
+  "type": "object",
+  "properties": {
+    "goal": { "type": "string" },
+    "stages": {
+      "type": "array",
+      "items": {
+        "type": "object",
+        "properties": {
+          "id": { "type": "string" },
+          "role": { "type": "string" },
+          "prompt": { "type": "string" },
+          "needs": { "type": "array", "items": { "type": "string" } },
+          "produces": { "type": "string" },
+          "tools": { "type": "array", "items": { "type": "string" } }
+        },
+        "required": ["id", "prompt", "produces"]
+      }
+    },
+    "edges": {
+      "type": "array",
+      "items": {
+        "type": "object",
+        "properties": {
+          "from": { "type": "string" },
+          "to": { "type": "string" },
+          "condition": {
+            "type": "object",
+            "properties": {
+              "type": { "type": "string" },
+              "artifact_id": { "type": "string" },
+              "field": { "type": "string" },
+              "value": { "type": "string" },
+              "operator": { "type": "string" }
+            },
+            "required": ["type"]
+          }
+        },
+        "required": ["from", "to", "condition"]
+      }
+    }
+  },
+  "required": ["goal", "stages", "edges"],
+  "additionalProperties": true
+}