feat(vcs): auto-rebase and retry a push when the remote moved ahead

[enisze]

Problem

When pushing, if the remote branch had commits the local branch didn't, GitVcsDriver.pushCurrentBranch failed hard with "The remote branch has commits this branch doesn't. Pull or rebase onto the remote branch, then push again." — leaving the user to fetch, rebase, and push by hand every time.

Fix

The push now self-heals the common non-fast-forward case. When a push is rejected only because the remote moved ahead, the driver:

1. Fetches the exact remote branch it's pushing to (git fetch <remote> <branch>).
2. Rebases the local commits onto it (git rebase FETCH_HEAD).
3. Retries the push once.

Safety:

• A rebase that hits conflicts is aborted (git rebase --abort) so the working tree is never left mid-rebase, and the conflict is surfaced with an actionable message.
• Only non-fast-forward rejections are retried — auth failures, missing workflow scope, and branch-protection rejections still fail fast, unchanged.
• The retry is single-shot (no further auto-rebase), so a second rejection fails normally.

Applies to all remote-targeted push paths: existing upstream, new upstream (-u), and explicit remoteName.

Tests

Added two integration tests in GitVcsDriverCore.test.ts (real git, real temp remotes):

• auto-rebases onto the remote and retries when the remote moved ahead (both commits end up on the remote, history linear).
• aborts the auto-rebase and leaves a clean tree when it conflicts (not mid-rebase, local commit intact, actionable error).

All 56 tests in the file pass.

🤖 Generated with Claude Code