feat: add consistent JSON output across the CLI

[rmanibus]

Summary

• add a shared global -json flag for CLI result output
• emit structured JSON for operational commands instead of human summaries when requested
• update command help, completion, tests, and the user guide to reflect the new behavior

Validation

• env GOCACHE=/tmp/cloudstic-gocache GOMODCACHE=/tmp/cloudstic-gomodcache go test -count=1 ./cmd/cloudstic
• env GOCACHE=/tmp/cloudstic-gocache GOMODCACHE=/tmp/cloudstic-gomodcache go test -count=1 ./internal/engine -run 'TestBackupManager_Run|TestFindPreviousSnapshot_'
• env GOCACHE=/tmp/cloudstic-gocache GOMODCACHE=/tmp/cloudstic-gomodcache golangci-lint run ./...

Closes #187

[codecov]

Codecov Report

❌ Patch coverage is 48.27586% with 45 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/cloudstic/json_output.go	54.54%	14 Missing and 1 partial ⚠️
cmd/cloudstic/cmd_forget.go	0.00%	5 Missing and 2 partials ⚠️
cmd/cloudstic/cmd_key.go	0.00%	6 Missing ⚠️
cmd/cloudstic/cmd_restore.go	0.00%	4 Missing and 2 partials ⚠️
cmd/cloudstic/cmd_check.go	50.00%	2 Missing and 1 partial ⚠️
cmd/cloudstic/cmd_backup.go	0.00%	1 Missing and 1 partial ⚠️
cmd/cloudstic/cmd_init.go	0.00%	1 Missing and 1 partial ⚠️
cmd/cloudstic/cmd_ls.go	0.00%	1 Missing and 1 partial ⚠️
cmd/cloudstic/cmd_prune.go	0.00%	1 Missing and 1 partial ⚠️

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