graph.json: add a schema version so external tools can read the format safely

We are building an integration in Bastra Recall (an open-source memory layer for AI coding agents) that reads graphify's graph.json, so an agent can see which parts of a codebase depend on a file before it changes it.

While doing that we noticed that graph.json carries no format version ("graph": {}). An external tool can only detect a format change by fields that are missing or renamed.

Proposal: a graph.schema_version field that is bumped on incompatible changes, and optionally the graphify version that produced the file. This fits with #2077 (recording the backend and model in graph.json).

If this sounds good to you, we're happy to send a PR.