Add git sync check contract

rules/patterns/git-sync-check/contract.md

@@ -0,0 +1,22 @@
+# Git Sync Check
+
+## Rule
+
+Before starting any work on a task, check whether the remote repository has commits that are not yet present locally.
+
+## Required Steps
+
+1. Run `git fetch` to update remote-tracking refs without merging.
+2. Check for upstream commits: `git log HEAD..@{u} --oneline`.
+3. If the output is non-empty (there are new remote commits):
+   - **Stop immediately. Do not make any changes.**
+   - Inform the user that the remote has new commits and ask how to proceed (e.g., pull, rebase, or ignore).
+4. If the output is empty, proceed with the task normally.
+
+## Rationale
+
+Working on an outdated local state risks merge conflicts, duplicate work, and overwriting changes made by other contributors. Checking remote state first keeps the working tree aligned and prevents avoidable conflicts.
+
+## Exceptions
+
+- Offline environments where `git fetch` is not possible: notify the user that the check could not be performed before proceeding.