ci: pin checkout SHA; always checkout z-shell/zd in reusable workflow

[ss-o]

Problem

When test-native.yml is called as a reusable workflow from another repo (e.g. z-shell/zi), actions/checkout without an explicit repository: checks out the calling repository, not z-shell/zd. This means the tests/ directory (which lives in zd) is not present, and ZUnit fails with Test file or directory not found.

Fix

• Pin actions/checkout to its v4 SHA
• Add repository: z-shell/zd so the test infrastructure is always available regardless of which repo calls this workflow

The zi repository is still cloned separately via the Install zi step using zi_repo/zi_ref inputs.

[Copilot]

Pull request overview

Pins actions/checkout to a specific v4 SHA and explicitly checks out z-shell/zd so the reusable workflow has access to the tests/ directory when invoked from other repositories (e.g. z-shell/zi).

Changes:

• Pin actions/checkout action to commit SHA for v4
• Add explicit repository: z-shell/zd to ensure test infrastructure is checked out regardless of caller

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