chore: extract shared profile builder helpers

[rmanibus]

Summary

• Extract shared profile-builder helpers for ref-name validation, profiles config loading/initialization, and sorted key utilities used by profile, auth, and store flows.
• Refactor store new by moving existing-store prefill/default application and profile-store object construction into dedicated helpers to reduce command-level complexity.
• Keep command behavior and CLI surface unchanged while reducing duplication and making future builder UX work safer to evolve.

Testing

• go test -count=1 ./cmd/cloudstic
• go test -count=1 ./...
• golangci-lint run ./...

[codecov]

Codecov Report

❌ Patch coverage is 79.33333% with 31 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/cloudstic/store_builder_helpers.go	70.14%	10 Missing and 10 partials ⚠️
cmd/cloudstic/cmd_auth.go	62.50%	3 Missing ⚠️
cmd/cloudstic/cmd_store.go	91.89%	3 Missing ⚠️
cmd/cloudstic/profile_builder_helpers.go	89.65%	2 Missing and 1 partial ⚠️
cmd/cloudstic/cmd_profile.go	77.77%	2 Missing ⚠️

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