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)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2240).
* #2264
* #2263
* __->__ #2240

codex-rs/tui/src/chatwidget.rs

@@ -1,5 +1,6 @@
 use std::collections::HashMap;
 use std::path::PathBuf;
+use std::sync::Arc;
 
 use codex_core::config::Config;
 use codex_core::parse_command::ParsedCommand;
@@ -54,6 +55,7 @@ mod agent;
 use self::agent::spawn_agent;
 use crate::streaming::controller::AppEventHistorySink;
 use crate::streaming::controller::StreamController;
+use codex_core::ConversationManager;
 use codex_file_search::FileMatch;
 
 // Track information about an in-flight exec command.
@@ -498,12 +500,13 @@ impl ChatWidget<'_> {
 
     pub(crate) fn new(
         config: Config,
+        conversation_manager: Arc<ConversationManager>,
         app_event_tx: AppEventSender,
         initial_prompt: Option<String>,
         initial_images: Vec<PathBuf>,
         enhanced_keys_supported: bool,
     ) -> Self {
-        let codex_op_tx = spawn_agent(config.clone(), app_event_tx.clone());
+        let codex_op_tx = spawn_agent(config.clone(), app_event_tx.clone(), conversation_manager);
 
         Self {
             app_event_tx: app_event_tx.clone(),