docs(ai_client): document Result API + deprecation

docs/guide_ai_client.md

@@ -174,7 +174,13 @@ ai_client.clear_comms_log()  # Clear
 ai_client.get_token_stats(md_content)  # Estimate token usage
 ```
 
-### Provider Error Taxonomy
+### Provider Error Taxonomy — Legacy (Pre-Refactor)
+
+> **As of 2026-06-11:** This section describes the pre-refactor exception-based
+> pattern. The `ProviderError` class is **removed** in the
+> `data_oriented_error_handling_20260606` track. See the new
+> [Data-Oriented Error Handling (Fleury Pattern)](#data-oriented-error-handling-fleury-pattern)
+> section below for the current convention.
 
 ```python
 class ProviderError(Exception):
@@ -186,7 +192,12 @@ class ProviderError(Exception):
         """Returns a user-friendly error message."""
 ```
 
-`ProviderError` is raised by provider-specific `_send_*` functions on failure. The caller (typically `app_controller.py`) catches it and surfaces the error to the user via `app.ai_status`.
+`ProviderError` was raised by provider-specific `_send_*` functions on failure.
+The caller (typically `app_controller.py`) caught it and surfaced the error to
+the user via `app.ai_status`. Post-refactor, the same flow uses `ErrorInfo`
+dataclasses inside `Result[str]` returns — see the new section below.
+
+---
 
 ---
 
@@ -426,6 +437,81 @@ def test_send_routes_to_provider(monkeypatch):
 
 Gated by env var (e.g., `RUN_REAL_AI_TESTS=1`). Hits the real API. Not in default CI.
 
+## Data-Oriented Error Handling (Fleury Pattern)
+
+The provider layer follows the "errors are just cases" framework
+(Ryan Fleury, [The Easiest Way To Handle
+Errors](https://www.dgtlgrove.com/p/the-easiest-way-to-handle-errors)). The
+canonical reference is
+[`conductor/code_styleguides/error_handling.md`](../conductor/code_styleguides/error_handling.md).
+
+### Result-Based Returns
+
+All `_send_<vendor>_result()` functions (8 vendors: Gemini, Anthropic,
+DeepSeek, MiniMax, Gemini CLI, Qwen, Llama, Grok — plus the
+`_send_llama_native` Ollama adapter) return `Result[str, ErrorInfo]`. SDK
+exceptions are caught at the boundary (`src/openai_compatible.py`,
+`src/qwen_adapter.py`) and converted to `ErrorInfo` dataclasses. The
+`_classify_<vendor>_error()` functions return `ErrorInfo` (not raise
+`ProviderError`, which has been removed).
+
+The 12 canonical `ErrorKind` values: `NETWORK`, `AUTH`, `QUOTA`,
+`RATE_LIMIT`, `BALANCE`, `PERMISSION`, `NOT_FOUND`, `INVALID_INPUT`,
+`NOT_READY`, `UNKNOWN`, `CONFIG`, `INTERNAL`. Each has exactly one
+meaning — do not overload `UNKNOWN` when a new failure mode surfaces
+(Lottes's anti-pattern). `ErrorInfo.source` is one of
+`"ai_client.<vendor>"` (e.g., `"ai_client.gemini"`,
+`"ai_client.anthropic"`) for diagnostic routing.
+
+### Public API
+
+- **`ai_client.send_result(...)`** — the new public API. Returns
+  `Result[str, ErrorInfo]`. Mirrors the `send()` signature (13+
+  parameters including 8 callbacks). Internally calls
+  `_send_<vendor>_result()` for the active provider.
+- **`ai_client.send(...)`** — **deprecated.** Emits `DeprecationWarning`
+  at runtime (via `typing_extensions.deprecated`; cached per call site to
+  avoid log spam). Returns `str` (the response text) for backward compat.
+  Errors are logged to the comms log via the deprecated path's comms entry
+  but not returned. Will be removed in the `public_api_migration_20260606`
+  follow-up track.
+
+### Example
+
+```python
+from src import ai_client
+from src.result_types import ErrorKind
+
+r = ai_client.send_result("system prompt", "user message")
+if not r.ok:
+    for err in r.errors:
+        log.error(err.ui_message())
+        # err.kind is one of ErrorKind.*; err.source is "ai_client.<vendor>"
+# use r.data regardless (it's the zero-initialized "" on failure)
+print(r.data)
+```
+
+### Migration Notes for Existing Callers
+
+- The `app_controller._api_generate` path and the MMA worker dispatch
+  (`multi_agent_conductor.py:591`) call `ai_client.send()`. They will
+  continue to work during the deprecation window; migration to
+  `send_result()` is the work of the `public_api_migration_20260606`
+  follow-up track.
+- Tests that mock `ai_client._send_<vendor>` should be updated to mock
+  `_send_<vendor>_result()` (or `send_result()` at the public API level).
+- `tests/conftest.py` adds a `filterwarnings` entry to silence the
+  `DeprecationWarning` from `send()` during the transition; new tests
+  for the new API should assert the warning is **not** emitted by
+  `send_result()`.
+
+### See Also (in-doc)
+
+- [`conductor/code_styleguides/error_handling.md`](../conductor/code_styleguides/error_handling.md) — canonical styleguide (5 patterns, data model, decision tree, anti-patterns)
+- [`conductor/tracks/data_oriented_error_handling_20260606/spec.md`](../conductor/tracks/data_oriented_error_handling_20260606/spec.md) — the spec that introduced this pattern
+- [`docs/guide_mcp_client.md`](guide_mcp_client.md#data-oriented-error-handling-fleury-pattern) — same pattern in the MCP tool layer
+- [`docs/guide_rag.md`](guide_rag.md#data-oriented-error-handling-fleury-pattern) — same pattern in the RAG engine
+
 ---
 
 ## See Also