fix(trunk): anchor git-diff-check at the workspace root

[ss-o]

Summary

git-diff-check failed on every run in this repository with:

fatal: not a git repository: (null)

Trunk copies lint targets into a sandbox directory (/tmp/trunk-<uid>/…) and runs the linter from there. This repository is consumed as a submodule of the Z-Shell meta-workspace, so its .git is a file containing a gitdir path relative to the superproject:

gitdir: ../../../.git/modules/…

That relative path does not resolve from the sandbox, so git diff --check could not find a repository and exited 128 on every target.

Fix

Anchor the command at the real workspace so repository discovery resolves against the actual checkout:

lint:
  definitions:
    - name: git-diff-check
      commands:
        - name: lint
          run: git -C "${workspace}" diff --check ${target}

This is the override documented for relative-gitdir checkouts; the upstream plugin definition is otherwise unchanged.

Scope

The same failure affected 9 repositories in the workspace. Sibling PRs carry the identical change. z-shell/zsh-lint was checked and deliberately not patched — its gitdir is absolute, so it was never affected.

Test plan

• trunk check --no-fix --ci --filter=git-diff-check <file> returns ✔ No issues (previously exit 128)
• trunk check --no-fix --ci <file> reports no not a git repository errors and no missing binaries
• trunk.yaml parses with exactly one lint.definitions key and one git-diff-check entry (guards against a duplicate-key merge that would silently drop other custom linter definitions)

Note for reviewers

CI runners clone normally, so .git is a real directory there and this check was already passing in CI. The bug only reproduces in a submodule/worktree checkout — which is how the meta-workspace consumes this repo. The override is a no-op for the CI case.