feat(index): default-ignore dependency/build/cache dirs (#407) (#417)

node_modules and other dependency/build/cache dirs were indexed whenever a project lacked a .gitignore excluding them (common in non-git projects), flooding context/search with third-party symbols. Added a built-in default-ignore set spanning every supported language/framework (node_modules, vendor, dist, build, target, .venv, __pycache__, Pods, .next, etc.), applied UNIFORMLY in both the git and non-git enumeration paths — including to tracked files, since committing/vendoring a dependency dir doesn't make it project code. The only opt-in is an explicit .gitignore negation (e.g. !vendor/). First-party-prone names (packages, lib, app, bin, src, deps, env) are deliberately excluded from the list.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>