[rollout_trace] Add debug trace reduction command (#18880)

## Summary

Adds the debug CLI entry point for reducing recorded rollout traces.
This gives developers a direct way to inspect whether the emitted trace
stream reduces into the expected conversation/runtime model.

## Stack

This is PR 5/5 in the rollout trace stack.

- [#18876](https://github.com/openai/codex/pull/18876): Add rollout
trace crate
- [#18877](https://github.com/openai/codex/pull/18877): Record core
session rollout traces
- [#18878](https://github.com/openai/codex/pull/18878): Trace tool and
code-mode boundaries
- [#18879](https://github.com/openai/codex/pull/18879): Trace sessions
and multi-agent edges
- [#18880](https://github.com/openai/codex/pull/18880): Add debug trace
reduction command

## Review Notes

This PR is intentionally last: it depends on the trace crate, core
recorder, runtime/tool events, and session/agent edge data all existing.
The command should remain a debug/developer tool and avoid adding new
runtime behavior.

The useful review question is whether the CLI exposes the reducer in the
smallest practical way for local inspection without turning the debug
command into a supported user-facing workflow.

codex-rs/core/src/tools/code_mode/execute_handler.rs

@@ -34,13 +34,16 @@ impl CodeModeExecuteHandler {
         // Allocate before starting V8 so the trace can create the parent
         // CodeCell before model-authored JavaScript issues nested tool calls.
         let runtime_cell_id = exec.session.services.code_mode_service.allocate_cell_id();
-        let code_cell_trace = exec.session.services.rollout_trace.start_code_cell_trace(
-            exec.session.conversation_id,
-            exec.turn.sub_id.as_str(),
-            runtime_cell_id.as_str(),
-            call_id.as_str(),
-            args.code.as_str(),
-        );
+        let code_cell_trace = exec
+            .session
+            .services
+            .rollout_thread_trace
+            .start_code_cell_trace(
+                exec.turn.sub_id.as_str(),
+                runtime_cell_id.as_str(),
+                call_id.as_str(),
+                args.code.as_str(),
+            );
         let started_at = std::time::Instant::now();
         let response = exec
             .session

codex-rs/core/src/tools/code_mode/wait_handler.rs

@@ -85,12 +85,8 @@ impl ToolHandler for CodeModeWaitHandler {
                     };
                     exec.session
                         .services
-                        .rollout_trace
-                        .code_cell_trace_context(
-                            exec.session.conversation_id,
-                            exec.turn.sub_id.as_str(),
-                            runtime_cell_id,
-                        )
+                        .rollout_thread_trace
+                        .code_cell_trace_context(exec.turn.sub_id.as_str(), runtime_cell_id)
                         .record_ended(response);
                 }
                 handle_runtime_response(&exec, wait_response.into(), args.max_tokens, started_at)