Sync feature branch with dev before committing in /ship to prevent merge conflicts

[sebastientaggart]

Problem

When using /start → /ship sequentially on multiple tasks, merge conflicts consistently occur in generated files (e.g. adapter skill files produced by sync.sh).

The root cause: /ship commits and pushes without first syncing the feature branch with the latest dev. If dev has advanced (from a previously merged PR), the feature branch is stale, and the merge step fails with conflicts — even when the actual code changes are compatible.

Solution

Add a "sync with base branch" step in /ship between identifying the linked issue (Step 3) and committing (Step 4). This step runs git fetch origin <base> && git merge origin/<base> to bring the feature branch up to date before committing.

Using merge (not rebase) because it's safe in all cases: unpushed branches fast-forward cleanly, and branches with existing pushed commits get a clean merge commit without requiring force push.

Expected outcome

Sequential /start → /ship cycles no longer produce merge conflicts from stale feature branches.

[sebastientaggart]

Agent Implementation Notes

File to change: skills/ship.md

Approach: Insert a new step between current Step 3 (Identify linked issue) and Step 4 (Commit) that syncs the feature branch with the base branch.

The new step should:

1. Fetch the latest base branch (dev in multi-branch mode, main in trunk mode)
2. Merge it into the current feature branch using git merge (not rebase — safe for all cases)
3. If the merge has conflicts, stop and tell the user to resolve them before continuing

This uses the existing {{BRANCH_DEV}} / {{BRANCH_PROD}} template variables to determine the correct base branch, matching the conditional pattern already used in Steps 5 and 7.

All subsequent steps get renumbered (current Steps 4-7 become Steps 5-8).