chore: configure Claude Code datum plugins

[scotwells]

This PR configures Claude Code to automatically include the datum-cloud plugins from the shared plugin marketplace.

Changes

• Adds .claude/settings.json with the datum-cloud plugin marketplace configuration
• Enables both datum-platform and datum-gtm plugins by default

Settings Added

{
  "extraKnownMarketplaces": {
    "datum-claude-code-plugins": {
      "source": {
        "source": "github",
        "repo": "datum-cloud/claude-code-plugins"
      }
    }
  },
  "enabledPlugins": {
    "datum-platform@datum-claude-code-plugins": true,
    "datum-gtm@datum-claude-code-plugins": true
  }
}

🤖 Generated with Claude Code