docs: record the step-model split in the changelog (#517)

Author: hertznsk

CHANGELOG.md

@@ -41,6 +41,33 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   without replacing tool-based output, weakening schema validation, or
   changing authored system prompts.
 
+### Changed
+
+- **Step definitions split into concrete models with a discriminated union**
+  (#517) — the single monolithic `AgentDef` that carried every step type's
+  fields is now a set of focused Pydantic models: `AgentDef` (provider-backed
+  LLM agents only), `HumanGateStepDef`, `QuestionsStepDef`, `ScriptStepDef`,
+  `MCPStepDef`, `WaitStepDef`, `SetStepDef`, `TerminateStepDef`, and
+  `WorkflowStepDef`, united by the static `StepDef` union discriminated on
+  `type`. Every model owns exactly the fields meaningful for its kind with
+  `extra="forbid"`, so a misplaced field is rejected next to its step instead
+  of being silently ignored, and the published JSON Schema now exposes a
+  `oneOf` + `discriminator` mapping with per-variant
+  `additionalProperties: false` for editors and tooling. Compatibility:
+  workflow YAML is unchanged — an omitted `type` or an explicit `type: null`
+  still loads as an LLM agent (now canonicalized to `type: "agent"` at parse
+  time) — and `AgentDef(...)` keeps working for LLM agents, including
+  `AgentDef(type=None, ...)`. What changed: constructing a non-LLM step
+  programmatically via `AgentDef(type="script", ...)` no longer works — use
+  the named step class (e.g. `ScriptStepDef(...)`) — and a field that belongs
+  to a different step type now fails with Pydantic's standard
+  `extra_forbidden` error rather than the previous custom
+  `"<type> agents cannot have '<field>'"` messages. Human gates additionally
+  lost the inert `model` field and can no longer appear as a `for_each`
+  inline agent (concurrent iterations would compete for one interactive
+  channel; route to a gate from the group's `routes:` instead). All classes
+  are exported from `conductor.config` and `conductor.config.schema`.
+
 ## [0.1.37](https://github.com/microsoft/conductor/compare/v0.1.36...v0.1.37) - 2026-09-09
 
 ### Added