fix(cli): support '-' for stdin in backup restore

[Juqsi]

Description

zed backup restore - currently fails with unable to stat restore file: stat -: no such file or directory because the code attempts to call os.Stat() on the literal "-" string.

This PR adds standard Unix - shorthand support for stdin, aligning the behavior with zed backup create - (implemented in #688). openRestoreFile now treats filename == "-" identically to filename == "", returning os.Stdin directly.

Testing

• Added table-driven unit test TestOpenRestoreFile in backup_test.go covering "", -, valid file paths, and invalid file errors
• Manually verified via: cat backup.zed | zed backup restore -

References

• Closes zed backup restore fails when using '-' for stdin #698
• Relates to fix: write default backup to cwd and print saved path #688

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[Juqsi]

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[Juqsi]

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