Motivation

• Ensure callers can detect when update_direct_target encountered merge conflicts versus succeeded.
• Prevent attempting to update descendant branches when a direct target failed due to conflicts.
• Provide a clear log message to indicate descendants are skipped until conflicts are resolved.

Description

• Updated update-pr-stack.sh to make update_direct_target return 0 on success and 1 when merge conflicts are detected.
• Added an explicit return 0 after the successful custom-merge path and return 1 in the conflict handling path.
• Changed main to call update_branch_recursive only if update_direct_target returns success, otherwise it echoes a warning like ⚠️ Skipping descendants of $BRANCH until conflicts are resolved.
• Modified output and labeling behavior remains the same, and the change is contained to update-pr-stack.sh.

Testing

• No automated tests were run for this change.
• The repository changes were committed locally and the updated script file is staged and committed.
• Basic invocation paths were not executed in CI as part of this patch.
• Manual verification is recommended when running the script in a real repository to validate conflict and success flows.

Codex Task