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


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

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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/Cargo.toml

@@ -37,6 +37,9 @@ unicode-width = { workspace = true }
 owo-colors = { workspace = true, features = ["supports-colors"] }
 supports-color = { workspace = true }
 
+[dependencies.async-trait]
+workspace = true
+
 [dev-dependencies]
 async-trait = { workspace = true }
 pretty_assertions = { workspace = true }