Add session forking to codex exec (#37367)

## What changed

- Add `codex exec fork <SESSION_ID> [PROMPT]` for creating a new thread from
  an existing session ID or thread name.
- Allow creating the fork without starting a turn, or immediately continue it
  with a prompt and optional images.
- Preserve the source thread ID in the emitted session configuration while
  leaving the source session unchanged.

## Testing

- Add CLI parsing coverage and an end-to-end test for promptless and prompted
  forks, thread naming, copied history, and unsupported promptless options.

GitOrigin-RevId: b8f28b238a1526cc486c5beb886a63f68b8e7986

codex-rs/exec/src/cli_tests.rs

@@ -61,6 +61,30 @@ fn resume_accepts_output_flags_after_subcommand() {
     assert_eq!(args.prompt.as_deref(), Some(PROMPT));
 }
 
+#[test]
+fn fork_parses_prompt_after_global_flags() {
+    const PROMPT: &str = "continue on the fork";
+    let cli = Cli::parse_from([
+        "codex-exec",
+        "fork",
+        "session-123",
+        "--json",
+        "--model",
+        "gpt-5.2-codex",
+        "--skip-git-repo-check",
+        "--ephemeral",
+        PROMPT,
+    ]);
+
+    assert!(cli.json);
+    assert!(cli.ephemeral);
+    let Some(Command::Fork(args)) = cli.command else {
+        panic!("expected fork command");
+    };
+    assert_eq!(args.session_id, "session-123");
+    assert_eq!(args.prompt.as_deref(), Some(PROMPT));
+}
+
 #[test]
 fn parses_config_isolation_flags() {
     let cli = Cli::parse_from([