fix(installer): refuse symlinked destinations

[morluto]

Summary

• refuse destination paths that contain symbolic links before creating directories, copying files, or creating backups
• preflight every recursively copied destination so nested file and directory links cannot redirect writes
• add regression coverage for a symlinked Codex skills root and an existing prompt-file symlink

The installer now fails with a clear error instead of following the link. This intentionally treats symlink-managed destinations as unsupported rather than guessing whether a link is expected.

Validation

• confirmed the new regression test fails on 34d197b because the skills install writes through the link
• bash -n install.sh tests/test_symlink_destinations.sh
• bash tests/test_symlink_destinations.sh
• ./tests/run.sh
• independent exact-diff review: no actionable findings

Closes #16

[chatgpt-codex-connector]

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