refactor(index): one index-session module for index/watch/serve/brain/MCP setup #567

Problem. config → embedding backend → vector store → skip patterns is assembled by hand at each entry point: index::prepare_index_config (used by main.rs:740/778, serve.rs), re-inlined in commands/watch.rs:36-49 and main.rs:1170-1190. Concrete bugs: serve passes None as config path so --config is ignored; MCP handle_brain_search (tools.rs:1044) never calls resolve_backend; review.rs:175 skips only index_skip_files while indexing uses ignore.files + index_skip_files. The index --watch poll loop duplicates commands/watch.rs. #565 threaded skip patterns through each entry point individually.

Direction. A single session module owns config → backend → store → skip → root → incremental index → stats. The Index arm of main.rs becomes thin dispatch; drop the duplicate poll loop in favour of watch.

Depends on the project-root seam issue.

From the 2026-10 architecture review (deepening candidates, develop @ 147dd75).