fix(agents): make librarian failures structured and user-safe

- run_librarian / run_librarian_stream raise AgentError instead of
  returning/yielding error text as normal output; detail stays in logs
- delegate_to_* wrappers now put a curated butler-toned sentence in
  DelegationResult.output on failure and never expose str(e), so
  streaming's error branch is reachable and honest
- _execute_single_delegation propagates success; direct delegation only
  records delegate_to_* as called when the expert actually succeeded
- librarian tools return user-safe messages instead of
  'Error searching: {e}' strings that leaked internal URLs into
  synthesis; coordination stream errors are curated as well
- ruff cleanups (TYPE_CHECKING forward refs, B904, unused locals) in
  the touched files to keep them lint-clean

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/agents/delegation.py

@@ -8,12 +8,12 @@ returns a structured result for synthesis.
 This implements the agent-as-tool pattern recommended by PydanticAI:
 agents call other agents via tool wrappers, keeping each agent focused.
 """
+from collections.abc import AsyncGenerator
 from dataclasses import dataclass, field
 from enum import Enum
-from typing import AsyncGenerator, Callable, Optional, Any
 
 from src.core.logging_config import get_logger
-from src.core.tracing import trace_span, SpanType
+from src.core.tracing import SpanType, trace_span
 
 logger = get_logger(__name__)
 
@@ -167,7 +167,7 @@ class DelegationTask:
     action: str = ""
     priority: int = 0
     depends_on: list[str] = field(default_factory=list)
-    result: Optional[str] = None
+    result: str | None = None
     task_id: str = ""
 
     def __post_init__(self):
@@ -186,14 +186,16 @@ class DelegationResult:
         expert_name: Which expert handled the task
         task: Original task description
         success: Whether the delegation succeeded
-        output: Expert's response/findings
-        error: Error message if failed
+        output: Expert's response/findings. On failure this holds a
+            curated, user-safe butler sentence (never exception detail)
+        error: Short user-safe error label if failed. Exception detail
+            stays in the logs only
     """
     expert_name: str
     task: str
     success: bool
     output: str
-    error: Optional[str] = None
+    error: str | None = None
 
 
 async def delegate_to_librarian(
@@ -285,12 +287,15 @@ async def delegate_to_librarian(
                 span.metadata["success"] = False
                 span.details["error"] = str(e)
 
+            # Exception detail stays in the logs; the user-facing output
+            # is a curated butler sentence so internals never leak into
+            # synthesis.
             return DelegationResult(
                 expert_name="librarian",
                 task=task,
                 success=False,
-                output="",
-                error=str(e),
+                output=get_think_message("librarian", task, "error"),
+                error="The Librarian was unable to complete the task.",
             )
 
 
@@ -383,12 +388,13 @@ async def delegate_to_biographer(
                 span.metadata["success"] = False
                 span.details["error"] = str(e)
 
+            # Exception detail stays in the logs only.
             return DelegationResult(
                 expert_name="biographer",
                 task=task,
                 success=False,
-                output="",
-                error=str(e),
+                output=get_think_message("biographer", task, "error"),
+                error="The Biographer was unable to complete the task.",
             )
 
 
@@ -480,12 +486,13 @@ async def delegate_to_housekeeper(
                 span.metadata["success"] = False
                 span.details["error"] = str(e)
 
+            # Exception detail stays in the logs only.
             return DelegationResult(
                 expert_name="housekeeper",
                 task=task,
                 success=False,
-                output="",
-                error=str(e),
+                output=get_think_message("housekeeper", task, "error"),
+                error="The Housekeeper was unable to complete the task.",
             )