feat: experimental --output-last-message flag to exec subcommand (#1037)

This introduces an experimental `--output-last-message` flag that can be
used to identify a file where the final message from the agent will be
written. Two use cases:

- Ultimately, we will likely add a `--quiet` option to `exec`, but even
if the user does not want any output written to the terminal, they
probably want to know what the agent did. Writing the output to a file
makes it possible to get that information in a clean way.
- Relatedly, when using `exec` in CI, it is easier to review the
transcript written "normally," (i.e., not as JSON or something with
extra escapes), but getting programmatic access to the last message is
likely helpful, so writing the last message to a file gets the best of
both worlds.

I am calling this "experimental" because it is possible that we are
overfitting and will want a more general solution to this problem that
would justify removing this flag.

codex-rs/exec/src/event_processor.rs

@@ -13,6 +13,7 @@ use codex_core::protocol::McpToolCallEndEvent;
 use codex_core::protocol::PatchApplyBeginEvent;
 use codex_core::protocol::PatchApplyEndEvent;
 use codex_core::protocol::SessionConfiguredEvent;
+use codex_core::protocol::TaskCompleteEvent;
 use owo_colors::OwoColorize;
 use owo_colors::Style;
 use shlex::try_join;
@@ -117,7 +118,9 @@ impl EventProcessor {
                 let msg = format!("Task started: {id}");
                 ts_println!("{}", msg.style(self.dimmed));
             }
-            EventMsg::TaskComplete => {
+            EventMsg::TaskComplete(TaskCompleteEvent {
+                last_agent_message: _,
+            }) => {
                 let msg = format!("Task complete: {id}");
                 ts_println!("{}", msg.style(self.bold));
             }