chore: reorganize cmd package

[rmanibus]

Summary

Reorganize the cmd/cloudstic package structure.

What Changes

• Internal command package/layout cleanup for maintainability.

Notes

• Refactor-oriented PR; behavior changes should be minimal.

[codecov]

Codecov Report

❌ Patch coverage is 62.56831% with 137 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/cloudstic/cmd_key.go	10.81%	33 Missing ⚠️
cmd/cloudstic/cmd_forget.go	56.45%	15 Missing and 12 partials ⚠️
cmd/cloudstic/cmd_ls.go	0.00%	17 Missing ⚠️
cmd/cloudstic/cmd_backup.go	72.72%	8 Missing and 4 partials ⚠️
cmd/cloudstic/cmd_restore.go	64.70%	9 Missing and 3 partials ⚠️
cmd/cloudstic/cmd_cat.go	62.50%	4 Missing and 2 partials ⚠️
cmd/cloudstic/main.go	57.14%	6 Missing ⚠️
cmd/cloudstic/cmd_check.go	75.00%	3 Missing and 2 partials ⚠️
cmd/cloudstic/cmd_diff.go	66.66%	4 Missing and 1 partial ⚠️
cmd/cloudstic/cmd_init.go	86.84%	4 Missing and 1 partial ⚠️
... and 4 more

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