fix(git): prevent reverting remote commits

[mp-hog]

Problem

Agent didn't check if there were new commits on remote, possibly overwriting them in the subsequent commit (for example leading to silently dropping CI artifacts).

Changes

• packages/git — assertNotBehindRemote: after fetching an existing branch, fail closed when the remote tip has commits the local checkout lacks. The error outputs the divergence and gives a work-preserving recovery (stash --include-untracked → fetch → reset --hard → stash pop). Mirrors the existing merge-in-progress / base-leak refusals; new and up-to-date branches are unaffected.
• packages/agent — documents the failure mode in the agent's git-workflow prompt and the tool description.

Tests

Real-git + bare-remote fixture reproducing the bug (commit → simulated bot push → second commit from a stale checkout → refused), plus up-to-date, local-ahead, and no-op (staged work preserved) cases. 39/39 pass.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit e8e3223.}

[greptile-apps]

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

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### Issue 1 of 2
packages/git/src/signed-commit.test.ts:263-284
**Redundant call + missed parameterisation opportunity**

The two `behindRemoteError` tests call the same pure function twice on slightly different inputs, which violates OnceAndOnlyOnce. The "stays generic" checks (absence of `"tests-posthog"` and `"OpenAPI"`) don't depend on a different input — they could run alongside the presence assertions on the same call. Collapsing to a single test, or using `it.each` if more combinations are ever added, would express the idea once.

### Issue 2 of 2
packages/git/src/signed-commit.test.ts:399-464
**Setup duplication across the last two `assertNotBehindRemote` tests**

The "no-op" and "local ahead" tests each hand-build their own bare-remote + agent fixture (~20 lines each) that reproduces almost exactly what `setupBranchAdvancedByBot` already does — just without the bot step. Extracting a `setupBaseRepo` helper (or passing an option to skip the bot) would satisfy the OnceAndOnlyOnce rule and make each test's intent clearer at a glance.

_{Reviews (1): Last reviewed commit: "fix(git): refuse signed commits that wou..." | Re-trigger Greptile}