fix: track index.html in CoGit; rename from _index.html

The context-tree navigation index was BM25-excluded (correct) and also
gitignored (wrong) — peers had to run `brv index rebuild` after every
pull to consume the latest index.

- Drop _index.html from CONTEXT_TREE_GITIGNORE_PATTERNS.
- Rename _index.html → index.html (drop the derived-file underscore
  prefix; it is now a tracked artifact).
- Special-case isExcludedFromSync so index.html participates in
  snapshot / merge / CoGit push while isDerivedArtifact still keeps
  it out of BM25 indexing and query results.

Existing users on this branch with a stale _index.html should delete
it; the next curate or `brv index rebuild` writes index.html.