feat(artifacts): config-driven custom artifact kinds with schema validation

Users declare artifact kinds in [[artifacts]] (id + schema_path to a JSON-schema file + llm_emitted); ConfigArtifactKind registers them at startup via createWorkflowLoader(extraKinds). New JsonSchemaValidator validates any non-built-in kind generically against its declared deriveJsonSchema(), so an LLM-emitted custom kind is checked against its shape, never trusted. Removed the dead payloadSerializer from the ArtifactKind contract. Schema source is path-to-file (not inline TOML — the hand-rolled parser can't nest).

docs/sample-config.toml

@@ -87,3 +87,13 @@ backend = "in_memory"          # or "turbovec"
 # script_path = "~/.config/correx/python/turbovec_sidecar.py"
 # dim = 1536
 # bit_width = 4
+
+# Custom artifact kinds. Each entry points to a JSON file holding the kind's JSON
+# schema (object type, typed properties, required, additionalProperties). Workflows
+# may then declare `produces` slots of this kind; LLM-emitted kinds are validated
+# against the declared schema. schema_path is absolute, ~-relative, or relative to
+# this config file's directory.
+# [[artifacts]]
+# id = "review_report"
+# schema_path = "schemas/review_report.json"
+# llm_emitted = true