refactor: remove internal Query-lane auto-parallelization

Remove the incorrect internal parallelization mechanism that automatically
ran Query-lane tools in parallel within a single LLM turn. This was
replaced by the correct External Task Handler pattern (coordinator/worker)
which provides opt-in parallel execution via set_lane_handler().

Removed:
- ParallelizationStrategy struct and enable_parallelization field (queue.rs)
- partition_by_lane(), should_use_parallel_execution(), execute_query_tools_parallel() (agent.rs)
- Parallelization fields from AgentLoop and AgentSession (agent.rs, agent_api.rs)
- PyParallelizationStrategy class (Python SDK)
- ParallelizationStrategy/enableParallelization (Node SDK)
- 8 example files: test_internal_parallel.rs/py/js and test_session_parallel_*.py
- docs/parallelization.md and crates/code/docs/parallelization_configuration.md
- partition_by_lane tests (5 tests removed)

Kept:
- External Task Handler pattern (set_lane_handler, pending_external_tasks, complete_external_task)
- Planning-mode step parallelization (JoinSet-based, unrelated)
- test_external_task_handler examples for all three SDKs

All 1133 lib tests pass, zero warnings.

Author: ZhiXiao-Lin

core/examples/test_external_task_handler.rs

@@ -109,7 +109,6 @@ async fn main() -> Result<()> {
 
     // 3. Stream a prompt that will trigger Execute-lane tools (bash)
     let start = Instant::now();
-    let session_clone = session.clone();
 
     let (mut rx, _handle) = session
         .stream(
@@ -131,7 +130,7 @@ async fn main() -> Result<()> {
                 println!("  📥 ExternalTaskPending: {} ({})", &task_id[..8], command_type);
 
                 // Poll all pending tasks
-                let tasks = session_clone.pending_external_tasks().await;
+                let tasks = session.pending_external_tasks().await;
                 for task in tasks {
                     println!("  🔧 Worker processing: {} → {}", task.command_type, &task.task_id[..8]);
 
@@ -153,7 +152,7 @@ async fn main() -> Result<()> {
                     }
 
                     // Complete the external task
-                    let completed = session_clone
+                    let completed = session
                         .complete_external_task(
                             &task.task_id,
                             ExternalTaskResult {