Align upgrade command spawning with create-astro

[matthewp]

Changes

• Replaces the shell-based command spawning in @astrojs/upgrade with the same Windows command resolution strategy used in create-astro.
• Routes known package manager shims through cmd.exe /d /s /c on Windows, while preserving direct execution for non-shim binaries like Bun.

Testing

• Installed dependencies in a fresh worktree to validate the split branch in isolation.
• Ran Biome against packages/upgrade/src/shell.ts and the new changeset to confirm the branch stays formatted and lint-clean.

Docs

• No docs update needed, because this only hardens internal command execution in @astrojs/upgrade.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5423beb

The changes in this PR will be included in the next version bump.

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