This PR formally integrates Google Cloud Spanner Graph as a first-class supported database backend for both the CodeGraphContext CLI and the internal MCP server.
Previously, cgc was strictly tied to Neo4j and Kuzu/FalkorDB. By introducing a native Spanner backend, developers can now securely host their CodeGraph indexes inside their GCP environment at enterprise scale while avoiding the maintenance overhead of a separate graph cluster.

Key Changes

• SpannerDBManager: Created core/database_spanner.py to seamlessly execute Graph operations.
• Dynamic Query Translation: Implemented a query middleware that intelligently maps standard Neo4j-style Cypher MERGE queries into native Spanner INSERT OR UPDATE SQL DML statements, ensuring DDL compliance while seamlessly routing MATCH read operations natively via Spanner GQL.
• CLI Setup Wizard: Fully replicated the Neo4j setup wizard for Spanner. Users can type cgc spanner setup (or cgc s) to authenticate using Application Default Credentials (ADC) and bind their GCP Project, Instance, and Database IDs locally. Spanner configuration will natively fall back to sensible defaults when environment variables are omitted.
• MCP Server Parity: Enabled dynamic database backend switching—the MCP server naturally inherits the CGC_RUNTIME_DB_TYPE=spanner configuration on boot and immediately begins routing file watch telemetry and structural graph analysis commands into the Spanner cluster.

Additional Bug Fixes

• Fixed visualize crashes: Addressed an issue caused by upstream commit fbe8b9e, where the visualizer commands and submodules were deleted from the codebase, but the corresponding import references inside cli/main.py were accidentally kept. This oversight was causing an immediate ModuleNotFoundError for anyone manually importing the codebase off the main branch. Removed the dangling imports and mock-disabled the visualizer shortcut cleanly so the CLI successfully boots again.

Verification

• Validated cgc spanner setup binds variables safely.
• Tested core schema generation and indexing operations safely against a live Spanner Graph instance via the CLI wrapper.
• Ensured that the default Neo4j fallback behavior isn't affected.