cloud: default to current branch in cloud exec (#7460)

## Summary
- add a shared git-ref resolver and use it for `codex cloud exec` and
TUI task submission
- expose a new `--branch` flag to override the git ref passed to cloud
tasks
- cover the git-ref resolution behavior with new async unit tests and
supporting dev dependencies

## Testing
- cargo test -p codex-cloud-tasks


------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_692decc6cbec8332953470ef063e11ab)

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Jeremy Rose <172423086+nornagon-openai@users.noreply.github.com>
Co-authored-by: Jeremy Rose <nornagon@openai.com>

codex-rs/cloud-tasks/src/cli.rs

@@ -34,10 +34,6 @@ pub struct ExecCommand {
     #[arg(long = "env", value_name = "ENV_ID")]
     pub environment: String,
 
-    /// Git branch to run in Codex Cloud.
-    #[arg(long = "branch", value_name = "BRANCH", default_value = "main")]
-    pub branch: String,
-
     /// Number of assistant attempts (best-of-N).
     #[arg(
         long = "attempts",
@@ -45,6 +41,10 @@ pub struct ExecCommand {
         value_parser = parse_attempts
     )]
     pub attempts: usize,
+
+    /// Git branch to run in Codex Cloud (defaults to current branch).
+    #[arg(long = "branch", value_name = "BRANCH")]
+    pub branch: Option<String>,
 }
 
 fn parse_attempts(input: &str) -> Result<usize, String> {