feat: C.1 - Add Google Cloud Secret Manager support

[mpyw]

Parent Issue

Part of #128

Summary

Add support for Google Cloud Secret Manager.

Prerequisites

• All Phase A issues (feat: A.1 - Define Scope type for multi-cloud support #132, feat: A.2 - Define Model types (Parameter, Secret) #134, feat: A.3 - Define Provider interfaces #137)
• All Phase B issues (feat: B.1 - Implement AWS Adapter #133, feat: B.2 - Add Scope support to Store #138, feat: B.3 - Update UseCase layer to use Model types #139)

CLI Commands

suve gcloud --project=my-project secret show my-secret
suve gcloud --project=my-project secret log my-secret
suve gcloud --project=my-project secret diff my-secret
suve gcloud --project=my-project secret list

# Staging
suve gcloud --project=my-project stage secret add my-secret
suve gcloud --project=my-project stage status
suve gcloud --project=my-project stage apply

# Alias
suve gcp --project=my-project secret show my-secret

Version Specification

Feature	Support
Version format	Integer (1, 2, 3...)
#N syntax	Yes
~N syntax	Yes
:LABEL syntax	No (not supported by Google Cloud)
State	enabled/disabled/destroyed

Directory Structure

internal/
├── provider/
│   └── gcloud/
│       └── secret/
│           ├── adapter.go
│           └── convert.go
├── version/
│   └── gcloudsecretversion/
│       └── parser.go
└── cli/
    └── commands/
        └── gcloud/
            ├── app.go
            └── secret/
                ├── show.go
                ├── log.go
                ├── diff.go
                └── list.go

Project Resolution

func resolveProject(flag string) (string, error) {
    if flag != "" {
        return flag, nil
    }
    if v := os.Getenv("CLOUDSDK_CORE_PROJECT"); v != "" {
        return v, nil
    }
    if v := os.Getenv("GOOGLE_CLOUD_PROJECT"); v != "" {
        return v, nil
    }
    out, err := exec.Command("gcloud", "config", "get-value", "project").Output()
    if err == nil && len(out) > 0 {
        return strings.TrimSpace(string(out)), nil
    }
    return "", errors.New("--project required")
}

Scope

• Implement internal/provider/gcloud/secret/ adapter
• Add version spec parser for Google Cloud
• Add gcloud command with gcp alias
• Implement staging support
• Add E2E tests with emulator

References

• Google Cloud Secret Manager Go SDK
• gcp-secret-manager-emulator