Fix zsh store completion definition

[rmanibus]

Summary

• fix the released zsh completion bug caused by an invalid -store option definition
• replace the malformed zsh store spec with a valid value completion restricted to local:, s3:, b2:, and sftp://
• update the zsh completion test to lock in the valid store completion spec

Testing

• go test -count=1 ./cmd/cloudstic -run '^TestCompletionZsh$'

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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