feat: implement configurable execution pipeline with Step protocol

- Add pipeline types: Step, BaseStep, StepResult, PipelineContext, PipelineTrace,
  ExecutionStrategy, StrategyInfo, PipelineEngine
- Extract 11 hardcoded executor steps into BuiltinStep classes
- Add build_standard/internal/testing/performance_strategy() factories
- Add Executor.strategy parameter (backward compatible, optional)
- Add call_with_trace() / call_async_with_trace() methods
- Add register_strategy(), list_strategies(), describe_pipeline() introspection

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Signed-off-by: tercel <tercel.yi@gmail.com>

Author: tercel

src/apcore/__init__.py

@@ -180,6 +180,24 @@
 # Events
 from apcore.events import A2ASubscriber, ApCoreEvent, EventEmitter, EventSubscriber, WebhookSubscriber
 
+# Pipeline
+from apcore.pipeline import (
+    BaseStep,
+    ExecutionStrategy,
+    PipelineAbortError,
+    PipelineContext,
+    PipelineTrace,
+    Step,
+    StepNameDuplicateError,
+    StepNotFoundError,
+    StepNotRemovableError,
+    StepNotReplaceableError,
+    StepResult,
+    StepTrace,
+    StrategyInfo,
+    StrategyNotFoundError,
+)
+
 # Trace Context
 from apcore.trace_context import TraceContext, TraceParent
 
@@ -511,6 +529,21 @@ def enable(module_id: str, reason: str = "Enabled via APCore client") -> dict[st
     # Version
     "VersionIncompatibleError",
     "negotiate_version",
+    # Pipeline
+    "Step",
+    "BaseStep",
+    "StepResult",
+    "PipelineContext",
+    "PipelineTrace",
+    "StepTrace",
+    "ExecutionStrategy",
+    "StrategyInfo",
+    "PipelineAbortError",
+    "StepNotFoundError",
+    "StepNotRemovableError",
+    "StepNotReplaceableError",
+    "StepNameDuplicateError",
+    "StrategyNotFoundError",
     # System Modules
     "register_sys_modules",
     "register_subscriber_type",