feat: CLI store configuration groundwork

[rmanibus]

Summary

Lay groundwork for CLI-based store configuration and related flag/help consolidation.

What Changes

• Store/flags/usage/completion updates in cmd/cloudstic.
• Docs updates reflecting revised store configuration behavior.
• Supporting test and script adjustments.

Tracking

• Related epic: RFC 0010: Epic / Tracking issue for backup profiles #96
• Milestone: RFC 0010: Backup profiles

Notes

• This PR establishes CLI config foundations used by subsequent profile/store work.

[codecov]

Codecov Report

❌ Patch coverage is 67.31518% with 84 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/cloudstic/usage.go	0.00%	44 Missing ⚠️
cmd/cloudstic/cmd_backup.go	54.83%	13 Missing and 1 partial ⚠️
cmd/cloudstic/store.go	88.23%	6 Missing and 6 partials ⚠️
cmd/cloudstic/cmd_forget.go	8.33%	10 Missing and 1 partial ⚠️
internal/engine/policy.go	60.00%	1 Missing and 1 partial ⚠️
cmd/cloudstic/cmd_init.go	66.66%	1 Missing ⚠️

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