fix: remove --project/--location flags, vertex top-level config, and GEOPENPROXY references

[Copilot]

Summary

This PR addresses several cleanup items identified in the codebase:

Changes

Remove --project and --location CLI flags

• These were GCP-specific parameters that added confusion since the app is not GCP-specific
• Removed from main.go flag parsing and printHelp()
• Removed Project and Location fields from the Flags struct in config.go

Remove vertex: top-level config section

• The Vertex VertexConfig field in Config is now tagged yaml:"-" so it is no longer loaded from the YAML config file
• Project/location are still populated from GCP environment variables (GOOGLE_CLOUD_PROJECT, CLOUDSDK_CORE_PROJECT, GOOGLE_CLOUD_LOCATION, etc.) for backward compatibility
• The internal VertexBaseURL() method continues to work for GCP backends

Fix startup failure when project is not configured

• Removed the validation check that returned an error (--project is required) when no GCP project was set
• The app now starts successfully without any project configured; individual backend requests that require a project will fail at request time rather than preventing startup

Fix GEOPENPROXY → LOCALMODELPROXY references

• docs/USAGE.md had three references to GEOPENPROXY_CONFIG (the prototype name) — all updated to LOCALMODELPROXY_CONFIG

Remove project/location from startup output

• Removed project= and location= from the plain/jsonl startup log line in ui.go
• Removed the Project/Location row from the TUI display

Tests

• Updated config tests to remove use of removed Flags.Project and vertex: YAML
• Added TestLoadsWithoutProject to explicitly verify the app starts when no project is configured
• All existing tests pass