Remove legacy codex exec --json format (#4525)

`codex exec --json` now maps to the behavior of `codex exec
--experimental-json` with new event and item shapes.

Thread events:
- thread.started
- turn.started
- turn.completed
- turn.failed
- item.started
- item.updated
- item.completed

Item types: 
- assistant_message
- reasoning
- command_execution
- file_change
- mcp_tool_call
- web_search
- todo_list
- error

Sample output:

<details>
`codex exec "list my assigned github issues"  --json | jq`

```
{
  "type": "thread.started",
  "thread_id": "01999ce5-f229-7661-8570-53312bd47ea3"
}
{
  "type": "turn.started"
}
{
  "type": "item.completed",
  "item": {
    "id": "item_0",
    "item_type": "reasoning",
    "text": "**Planning to list assigned GitHub issues**"
  }
}
{
  "type": "item.started",
  "item": {
    "id": "item_1",
    "item_type": "mcp_tool_call",
    "server": "github",
    "tool": "search_issues",
    "status": "in_progress"
  }
}
{
  "type": "item.completed",
  "item": {
    "id": "item_1",
    "item_type": "mcp_tool_call",
    "server": "github",
    "tool": "search_issues",
    "status": "completed"
  }
}
{
  "type": "item.completed",
  "item": {
    "id": "item_2",
    "item_type": "reasoning",
    "text": "**Organizing final message structure**"
  }
}
{
  "type": "item.completed",
  "item": {
    "id": "item_3",
    "item_type": "assistant_message",
    "text": "**Assigned Issues**\n- openai/codex#3267 – “stream error: stream disconnected before completion…” (bug) – last update 2025-09-08\n- openai/codex#3257 – “You've hit your usage limit. Try again in 4 days 20 hours 9 minutes.” – last update 2025-09-23\n- openai/codex#3054 – “reqwest SSL panic (library has no ciphers)” (bug) – last update 2025-09-03\n- openai/codex#3051 – “thread 'main' panicked at linux-sandbox/src/linux_run_main.rs:53:5:” (bug) – last update 2025-09-10\n- openai/codex#3004 – “Auto-compact when approaching context limit” (enhancement) – last update 2025-09-26\n- openai/codex#2916 – “Feature request: Add OpenAI service tier support for cost optimization” – last update 2025-09-12\n- openai/codex#1581 – “stream error: stream disconnected before completion: stream closed before response.complete; retrying...” (bug) – last update 2025-09-17"
  }
}
{
  "type": "turn.completed",
  "usage": {
    "input_tokens": 34785,
    "cached_input_tokens": 12544,
    "output_tokens": 560
  }
}
```

</details>

codex-rs/exec/src/lib.rs

@@ -1,9 +1,8 @@
 mod cli;
 mod event_processor;
 mod event_processor_with_human_output;
-pub mod event_processor_with_json_output;
+pub mod event_processor_with_jsonl_output;
 pub mod exec_events;
-pub mod experimental_event_processor_with_json_output;
 
 pub use cli::Cli;
 use codex_core::AuthManager;
@@ -22,8 +21,7 @@ use codex_core::protocol::TaskCompleteEvent;
 use codex_ollama::DEFAULT_OSS_MODEL;
 use codex_protocol::config_types::SandboxMode;
 use event_processor_with_human_output::EventProcessorWithHumanOutput;
-use event_processor_with_json_output::EventProcessorWithJsonOutput;
-use experimental_event_processor_with_json_output::ExperimentalEventProcessorWithJsonOutput;
+use event_processor_with_jsonl_output::EventProcessorWithJsonOutput;
 use opentelemetry_appender_tracing::layer::OpenTelemetryTracingBridge;
 use serde_json::Value;
 use std::io::IsTerminal;
@@ -59,7 +57,6 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         color,
         last_message_file,
         json: json_mode,
-        experimental_json,
         sandbox_mode: sandbox_mode_cli_arg,
         prompt,
         output_schema: output_schema_path,
@@ -212,17 +209,8 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         let _ = tracing_subscriber::registry().with(fmt_layer).try_init();
     }
 
-    let mut event_processor: Box<dyn EventProcessor> = match (json_mode, experimental_json) {
-        (_, true) => Box::new(ExperimentalEventProcessorWithJsonOutput::new(
-            last_message_file.clone(),
-        )),
-        (true, _) => {
-            eprintln!(
-                "The existing `--json` output format is being deprecated. Please try the new format using `--experimental-json`."
-            );
-
-            Box::new(EventProcessorWithJsonOutput::new(last_message_file.clone()))
-        }
+    let mut event_processor: Box<dyn EventProcessor> = match json_mode {
+        true => Box::new(EventProcessorWithJsonOutput::new(last_message_file.clone())),
         _ => Box::new(EventProcessorWithHumanOutput::create_with_ansi(
             stdout_with_ansi,
             &config,