chore: introduce ConversationManager as a clearinghouse for all conversations (#2240)

This PR does two things because after I got deep into the first one I
started pulling on the thread to the second:

- Makes `ConversationManager` the place where all in-memory
conversations are created and stored. Previously, `MessageProcessor` in
the `codex-mcp-server` crate was doing this via its `session_map`, but
this is something that should be done in `codex-core`.
- It unwinds the `ctrl_c: tokio::sync::Notify` that was threaded
throughout our code. I think this made sense at one time, but now that
we handle Ctrl-C within the TUI and have a proper `Op::Interrupt` event,
I don't think this was quite right, so I removed it. For `codex exec`
and `codex proto`, we now use `tokio::signal::ctrl_c()` directly, but we
no longer make `Notify` a field of `Codex` or `CodexConversation`.

Changes of note:

- Adds the files `conversation_manager.rs` and `codex_conversation.rs`
to `codex-core`.
- `Codex` and `CodexSpawnOk` are no longer exported from `codex-core`:
other crates must use `CodexConversation` instead (which is created via
`ConversationManager`).
- `core/src/codex_wrapper.rs` has been deleted in favor of
`ConversationManager`.
- `ConversationManager::new_conversation()` returns `NewConversation`,
which is in line with the `new_conversation` tool we want to add to the
MCP server. Note `NewConversation` includes `SessionConfiguredEvent`, so
we eliminate checks in cases like `codex-rs/core/tests/client.rs` to
verify `SessionConfiguredEvent` is the first event because that is now
internal to `ConversationManager`.
- Quite a bit of code was deleted from
`codex-rs/mcp-server/src/message_processor.rs` since it no longer has to
manage multiple conversations itself: it goes through
`ConversationManager` instead.
- `core/tests/live_agent.rs` has been deleted because I had to update a
bunch of tests and all the tests in here were ignored, and I don't think
anyone ever ran them, so this was just technical debt, at this point.
- Removed `notify_on_sigint()` from `util.rs` (and in a follow-up, I
hope to refactor the blandly-named `util.rs` into more descriptive
files).
- In general, I started replacing local variables named `codex` as
`conversation`, where appropriate, though admittedly I didn't do it
through all the integration tests because that would have added a lot of
noise to this PR.




---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2240).
* #2264
* #2263
* __->__ #2240

codex-rs/cli/src/proto.rs

@@ -1,15 +1,14 @@
 use std::io::IsTerminal;
-use std::sync::Arc;
 
 use clap::Parser;
 use codex_common::CliConfigOverrides;
-use codex_core::Codex;
-use codex_core::CodexSpawnOk;
+use codex_core::ConversationManager;
+use codex_core::NewConversation;
 use codex_core::config::Config;
 use codex_core::config::ConfigOverrides;
+use codex_core::protocol::Event;
+use codex_core::protocol::EventMsg;
 use codex_core::protocol::Submission;
-use codex_core::util::notify_on_sigint;
-use codex_login::CodexAuth;
 use tokio::io::AsyncBufReadExt;
 use tokio::io::BufReader;
 use tracing::error;
@@ -36,22 +35,38 @@ pub async fn run_main(opts: ProtoCli) -> anyhow::Result<()> {
         .map_err(anyhow::Error::msg)?;
 
     let config = Config::load_with_cli_overrides(overrides_vec, ConfigOverrides::default())?;
-    let auth = CodexAuth::from_codex_home(&config.codex_home)?;
-    let ctrl_c = notify_on_sigint();
-    let CodexSpawnOk { codex, .. } = Codex::spawn(config, auth, ctrl_c.clone()).await?;
-    let codex = Arc::new(codex);
+    // Use conversation_manager API to start a conversation
+    let conversation_manager = ConversationManager::default();
+    let NewConversation {
+        conversation_id: _,
+        conversation,
+        session_configured,
+    } = conversation_manager.new_conversation(config).await?;
+
+    // Simulate streaming the session_configured event.
+    let synthetic_event = Event {
+        // Fake id value.
+        id: "".to_string(),
+        msg: EventMsg::SessionConfigured(session_configured),
+    };
+    let session_configured_event = match serde_json::to_string(&synthetic_event) {
+        Ok(s) => s,
+        Err(e) => {
+            error!("Failed to serialize session_configured: {e}");
+            return Err(anyhow::Error::from(e));
+        }
+    };
+    println!("{session_configured_event}");
 
     // Task that reads JSON lines from stdin and forwards to Submission Queue
     let sq_fut = {
-        let codex = codex.clone();
-        let ctrl_c = ctrl_c.clone();
+        let conversation = conversation.clone();
         async move {
             let stdin = BufReader::new(tokio::io::stdin());
             let mut lines = stdin.lines();
             loop {
                 let result = tokio::select! {
-                    _ = ctrl_c.notified() => {
-                        info!("Interrupted, exiting");
+                    _ = tokio::signal::ctrl_c() => {
                         break
                     },
                     res = lines.next_line() => res,
@@ -65,7 +80,7 @@ pub async fn run_main(opts: ProtoCli) -> anyhow::Result<()> {
                         }
                         match serde_json::from_str::<Submission>(line) {
                             Ok(sub) => {
-                                if let Err(e) = codex.submit_with_id(sub).await {
+                                if let Err(e) = conversation.submit_with_id(sub).await {
                                     error!("{e:#}");
                                     break;
                                 }
@@ -88,8 +103,8 @@ pub async fn run_main(opts: ProtoCli) -> anyhow::Result<()> {
     let eq_fut = async move {
         loop {
             let event = tokio::select! {
-                _ = ctrl_c.notified() => break,
-                event = codex.next_event() => event,
+                _ = tokio::signal::ctrl_c() => break,
+                event = conversation.next_event() => event,
             };
             match event {
                 Ok(event) => {