fix: preserve target registry across workspace loads

[phasetr]

Summary:

• Preserve the wider target directory registry when loading workspace-local state.
• Preserve the registry for legacy workspace states that do not yet have :target-directories, while normalizing alist aliases to alias names.
• Report setup failures instead of printing a successful completion message.
• Add regression coverage for deleting a workspace and then setting up the deleted project alias again.

Tests:

• make check (264 tests, 264 passed)
• GitHub Actions CI passed previously; latest CI is running after removing AGENTS.md from the PR diff.

[github-actions]

🎉 This PR is included in version 0.19.4 🎉

The release is available on GitHub release

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