refactor: enforce BasedPyright strict type checking (#5158)

.agent/design.md

@@ -24,6 +24,14 @@ Fix bugs by changing only what is necessary. Do not bundle unrelated refactors o
 
 A bugfix should make the protected invariant clear, change the smallest surface that enforces it, and add only the closest regression test. If a diff starts changing ownership boundaries or mixing behavior changes with clean-up, split it before it becomes hard to review.
 
+## Type dynamic boundaries at the edge
+
+Wire payloads, persisted records, and third-party SDK objects are untrusted dynamic boundaries. Prefer a parser or small normalizer at the owning edge, and use `TypedDict` for stable dictionary shapes, so validation happens once and internal code receives a concrete type. Do not spread raw dynamic dictionaries or SDK objects through the core.
+
+Stable first-party dependencies must be typed where they are stored or passed. Do not declare an internal service, context field, or callback result as `Any` and then recover its real type with consumer-side casts. Use the concrete type or a narrow `Protocol`; reserve `Any` for genuinely dynamic boundaries.
+
+`typing.cast` performs no runtime validation. Every new cast must be supported by a runtime check on the same path or by an explicit invariant that is clear from construction and control flow (and documented locally when it is not obvious). If input can violate the claimed type, handle that invalid case before casting; never use `cast` only to silence BasedPyright.
+
 ## Explicit over magical
 
 Configuration must be declared explicitly in `config/schema.py` Pydantic models. Error handling should raise clear exceptions rather than silently correcting bad input. Provider auto-detection exists, but every resolution path must be traceable from the factory to the concrete provider class.