Add --rebase-on-conflict flag to `gh stack sync` to auto-run rebase on conflicts

Summary

Add a flag to gh stack sync so that when a conflict is detected it automatically runs gh stack rebase (instead of instructing the user to run rebase manually).

Background / motivation

Currently when users run gh stack sync and it encounters a conflict the CLI prints an instruction to run gh stack rebase. Some users would prefer a single-command flow that attempts a rebase automatically when a conflict occurs to avoid typing two commands.

Proposed behavior

• Add an explicit flag to gh stack sync, for example --rebase-on-conflict (alternative names: --rebase-if-conflict, --auto-rebase).
• Default behavior of gh stack sync remains unchanged (safe, no automatic rebase).
• When the flag is provided and sync detects a conflict, gh stack sync should automatically invoke the rebase flow (equivalent to running gh stack rebase).
• The flag signals that the user consents to the potential rebase-related outcomes (e.g., temporary detached HEAD during an in-progress rebase), so the command can proceed without an additional prompt.
• The command should preserve non-interactive behavior where appropriate (i.e., suitable for scripts). If prompts are required during rebase, consider a complementary --yes or --no-interactive option, or clearly document the interaction expectations.

Acceptance criteria

• gh stack sync --rebase-on-conflict will run a rebase automatically if a conflict is encountered during sync.
• The default gh stack sync remains unchanged and does not perform automatic rebases.
• Documentation for gh stack sync includes the new flag and describes its behavior and risks.

Example

• Normal: gh stack sync
• With flag: gh stack sync --rebase-on-conflict

Notes / implementation hints

• Consider the exact flag name and whether a short alias (e.g., -r) is appropriate.
• Ensure the UX communicates that the user opted in to automatic rebase behavior and any potential consequences.
• Make sure the behavior is deterministic for CI/automation usage (document or provide an explicit non-interactive mode).

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