graph: relative graphrag.index_path nests a second .agent-brain inside the state dir

[RichardHightower]

Summary

A relative graphrag.index_path in config.yaml resolves against the server process cwd, which is the state dir. index_path: .agent-brain/graph_index (the value the docs and the legacy ~/.agent-brain/config.yaml example use) therefore creates .agent-brain/.agent-brain/graph_index/.

Found during the v10.7.0 integration test on 2026-09-19.

Proposed fix

Resolve a relative graphrag.index_path against the project root, not the cwd, in agent-brain-server/agent_brain_server/config/provider_config.py::get_graphrag_config (or in _resolve_graph_index_path). Update config.yaml.example and the GraphRAG docs to show the default (graph_index under the state dir) and to say that relative paths are project-root relative. Add a test.

[RichardHightower]

Fixed in #257.

Relative graphrag.index_path resolves against the project root, not the state dir. The historical default graph_index / ./graph_index still lands in {state_dir}/data/graph_index. index_path: .agent-brain/graph_index no longer nests a second .agent-brain. Docs and config.yaml.example updated.