feat: add Secret Service secret backend

[rmanibus]

Closes #89

Summary

• add a native secret-service://collection/item backend using the Linux Secret Service D-Bus API
• distinguish missing collection/item from unavailable DBus or keyring session errors and include env://... fallback guidance for unavailable environments
• add unit and Linux-specific integration coverage that skips cleanly when Secret Service is unavailable

Testing

• go test ./internal/secretref
• go test ./cmd/cloudstic ./internal/secretref
• GOOS=linux GOARCH=amd64 go test -c ./internal/secretref

[codecov]

Codecov Report

❌ Patch coverage is 32.80000% with 84 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/secretref/secret_service_backend_linux.go	13.04%	79 Missing and 1 partial ⚠️
internal/secretref/secret_service_backend.go	86.20%	3 Missing and 1 partial ⚠️

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