Handle missing Git in optional repo checks

[charlespwd]

Why

Some theme workflows use Git only as an optional safety check before continuing. When Git is not installed or unavailable on PATH, those checks should not surface raw command failures to users.

What changed

• Treat missing Git as “not inside a Git directory” for optional repository detection.
• Keep a friendly, actionable error for flows that explicitly require Git.
• Added unit coverage for both behaviours.

Testing

• pnpm vitest run packages/cli-kit/src/public/node/git.test.ts --config packages/cli-kit/vite.config.ts
• pnpm vitest run packages/theme/src/cli/services/pull.test.ts --config packages/theme/vite.config.ts

Fixes shop/issues#47568

[Copilot]

Pull request overview

This PR adjusts the @shopify/cli-kit Git helpers so that optional repository checks don’t surface raw failures when git is missing from PATH, while flows that require Git still fail with a friendly, actionable error.

Changes:

• Made insideGitDirectory() return false when Git is not available (treat as “not a repo” for optional checks).
• Updated ensureInsideGitDirectory() to explicitly require Git first and then perform the repo check.
• Added unit tests covering both “Git missing” behaviors.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/cli-kit/src/public/node/git.ts	Separates “Git required” vs “optional repo detection” behavior by gating required checks with ensureGitIsPresentOrAbort() and making optional checks return false when Git is absent.
packages/cli-kit/src/public/node/git.test.ts	Adds unit tests ensuring missing-Git behavior is friendly for required flows and silent (false) for optional detection.

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[karreiro]

Thank you, @charlespwd!