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.

docs/schemas/execution_plan.json

@@ -12,6 +12,7 @@
           "prompt": { "type": "string" },
           "needs": { "type": "array", "items": { "type": "string" } },
           "produces": { "type": "string" },
+          "kind": { "type": "string" },
           "tools": { "type": "array", "items": { "type": "string" } }
         },
         "required": ["id", "prompt", "produces"]