feat: GCS bucket integration for team dashboard

[pauldatta]

Context

The team dashboard reads reports from a local folder. This issue adds GCS bucket as a source.

Scope

• Add --gcs gs://bucket/path/ flag to dashboard subcommand
• Use gcloud storage ls and gcloud storage cp to list and fetch manifest files
• Support incremental sync (only fetch new/modified files since last sync)
• Auto-detect available auth (gcloud auth or service account)

Acceptance Criteria

• npx gemini-cli-scanner dashboard --gcs gs://bucket/reports/ starts dashboard with GCS as source
• Engineers can upload via gcloud storage cp scan-results/gemini-env-manifest.json gs://bucket/reports/name-date.json

Related

• Depends on team dashboard v1 (local folder)
• See also: Drive folder integration (feat: Drive folder integration for team dashboard #7)