feat(cli): Add --project option to local-install and register monitoring instance

Summary

• Add --project flag to mon local-install for specifying project name
• Save project_name to .pgwatch-config for reporter to use
• Add registerMonitoringInstance() to call API after successful install
• Update postgres-reports.sh to read project_name from config file
• Add updatePgwatchConfig() helper to preserve existing config values

Usage

# With project name
postgres-ai mon local-install --project my-project --api-key <key>

# Default project name: postgres-ai-monitoring
postgres-ai mon local-install --api-key <key>

Project name priority

1. CLI --project flag
2. .pgwatch-config file (project_name=...)
3. Default: postgres-ai-monitoring

Test plan

• Run local-install with --project flag
• Verify .pgwatch-config contains project_name
• Verify API registration call (with DEBUG=1)
• Verify reporter reads project_name from config

🤖 Generated with Claude Code

Issue: https://gitlab.com/postgres-ai/platform/-/issues/380