feat: Drive folder integration for team dashboard

[pauldatta]

Context

The team dashboard (v1) reads reports from a local folder. This issue adds Google Drive as a source.

Scope

• Add --drive <folder-id> flag to dashboard subcommand
• Use Drive API or Workspace MCP to list and fetch manifest files from a shared Drive folder
• Support incremental sync (only fetch new/modified files since last sync)

Acceptance Criteria

• npx gemini-cli-scanner dashboard --drive <folder-id> starts dashboard with Drive as source
• Engineers can drop their gemini-env-manifest.json into a shared Drive folder
• Dashboard fetches and merges reports automatically

Related

• Depends on team dashboard v1 (local folder)
• Consider GCS bucket support in same PR