[exec] add include-plan-tool flag and print it nicely (#3461)

### Summary
Sometimes in exec runs, we want to allow the model to use the
`update_plan` tool, but that's not easily configurable. This change adds
a feature flag for this, and formats the output so it's human-readable

## Test Plan
<img width="1280" height="354" alt="Screenshot 2025-09-11 at 12 39
44 AM"
src="https://github.com/user-attachments/assets/72e11070-fb98-47f5-a784-5123ca7333d9"
/>

codex-rs/exec/src/cli.rs

@@ -67,6 +67,10 @@ pub struct Cli {
     #[arg(long = "json", default_value_t = false)]
     pub json: bool,
 
+    /// Whether to include the plan tool in the conversation.
+    #[arg(long = "include-plan-tool", default_value_t = false)]
+    pub include_plan_tool: bool,
+
     /// Specifies file where the last message from the agent should be written.
     #[arg(long = "output-last-message")]
     pub last_message_file: Option<PathBuf>,