feat: add writable native secret backends

[rmanibus]

Closes #141

Summary

• add shared writable support for wincred:// and secret-service:// backends through the internal/secretref writable backend contract
• implement native Windows Credential Manager writes and Linux Secret Service writes/existence checks using platform APIs
• extend backend and CLI tests so Windows and Linux interactive secret creation can build on the shared writable abstraction

Testing

• go test ./internal/secretref ./cmd/cloudstic
• GOOS=windows GOARCH=amd64 go test -c ./internal/secretref && GOOS=windows GOARCH=amd64 go test -c ./cmd/cloudstic
• GOOS=linux GOARCH=amd64 go test -c ./internal/secretref && GOOS=linux GOARCH=amd64 go test -c ./cmd/cloudstic
• golangci-lint run ./internal/secretref/... ./cmd/cloudstic/...

[codecov]

Codecov Report

❌ Patch coverage is 44.85981% with 59 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/secretref/secret_service_backend_linux.go	15.78%	32 Missing ⚠️
internal/secretref/secret_service_backend.go	65.62%	7 Missing and 4 partials ⚠️
internal/secretref/wincred_backend.go	65.62%	7 Missing and 4 partials ⚠️
internal/secretref/wincred_backend_stub.go	0.00%	5 Missing ⚠️

📢 Thoughts on this report? Let us know!