docs(product-guidelines): add Data-Oriented Error Handling section

conductor/product-guidelines.md

@@ -47,6 +47,51 @@
   - **Functions/Methods:** `[C: Caller1, Caller2]` (Primary callers).
   - **State Variables:** `[M: File:Line, Method]` (Mutation points) and `[U: File]` (Major use paths).
 
+## Data-Oriented Error Handling
+
+The codebase follows the "errors are just cases" framework from Ryan Fleury's
+[The Easiest Way To Handle Errors](https://www.dgtlgrove.com/p/the-easiest-way-to-handle-errors).
+The canonical reference (with code examples) is in
+[`conductor/code_styleguides/error_handling.md`](code_styleguides/error_handling.md).
+Key principles:
+
+- **Result dataclasses** instead of `Optional[T]` or exception-based control flow.
+- **Nil-sentinel dataclasses** instead of `None`.
+- **Zero-initialized fields** via `@dataclass` defaults.
+- **Fail early**: validation at the entry point, not deep in the call stack.
+- **AND over OR**: return a struct with data + side-channel errors, not a sum type.
+- **Exceptions reserved for the SDK boundary**: SDK errors are caught and converted
+  to `ErrorInfo` dataclasses; the rest of the application works with data, not control flow.
+
+This convention is established incrementally. The 2026-06-11
+`data_oriented_error_handling_20260606` track applies it to
+`src/mcp_client.py`, `src/ai_client.py`, and `src/rag_engine.py`. Future
+tracks will apply it to the remaining `src/` files
+(`src/app_controller.py`, `src/models.py`, `src/project_manager.py`, etc. —
+see `conductor/tracks/data_oriented_error_handling_20260606/spec.md` §12.2
+for the prioritized list).
+
+### `Optional[T]` ban (return types only)
+
+In the 3 refactored files (`src/mcp_client.py`, `src/ai_client.py`,
+`src/rag_engine.py`), `Optional[T]` return types are forbidden. Use
+`Result[T]` (with a `NIL_T` singleton if needed) instead. Argument types
+that may be `None` (e.g., `rag_engine: Optional[Any] = None`) remain
+allowed — they describe a caller choice, not a runtime failure of this
+function. The audit script `scripts/audit_optional_in_3_files.py` enforces
+this rule by failing CI on new `Optional[X]` return types in the 3
+refactored files.
+
+### Public API deprecation: `ai_client.send()` → `ai_client.send_result()`
+
+The public `ai_client.send()` is marked `@deprecated` (via
+`typing_extensions.deprecated`). It still works for backward compat but
+emits a `DeprecationWarning` at runtime. New code MUST use
+`ai_client.send_result()`, which returns `Result[str, ErrorInfo]` instead
+of `str`. Removal is planned in the follow-up
+`public_api_migration_20260606` track.
+
+</new_content>
 ## Testing Requirements
 
 These are the process standards the project's test infrastructure enforces. For the full implementation contract (fixture names, anti-patterns, audit scripts), see [docs/guide_testing.md §Structural Testing Contract](../docs/guide_testing.md) and the per-styleguide audit scripts in [code_styleguides/](code_styleguides/).