ref: make Node server cache-only, remove upstream fetching

The Node server used to call into per-package releases.js modules
(brew, fetcher, git-tag, gitea, github, githubish, etc.) on demand to
fetch and normalize upstream release data. With the Go cache daemon
(webicached) writing canonical JSON into ~/.cache/webi, the Node side
no longer needs to do any of that.

This commit removes the upstream-fetching path entirely:

- Delete _common/{brew,fetcher,git-tag,gitea,github,github-source,
  githubish,githubish-source}.js — the network-side fetchers.
- Delete _webi/normalize.js — the in-process normalization layer that
  fed transform-releases.js. Cache files are already normalized.
- Delete every <name>/releases.js (and _example/releases.js). The
  installer scripts that drive `webi <name>` no longer require a
  releases.js to be present; the cache file is the source of truth.
- Rewrite _webi/builds-cacher.js to read JSON from
  _cache/<yyyy-mm>/<name>.json instead of require()ing a releases
  module and invoking its latest()/network calls. Drops getLatestBuilds,
  the per-name promise coalescer, the freshener, and the
  process.nextTick refresh hook.
- Rewrite _webi/transform-releases.js to consume already-normalized
  cache entries (no normalize() call, no per-installer hooks).
- Update _webi/classify-one.js to read from the cache file directly.
- Update _webi/{builds.js,builds-cacher-test.js,lint-builds.js,test.js}
  to drop references to the removed modules.

Net effect: the Node server is a thin reader over the cache. webicached
owns refresh; the Node process never makes an upstream request.

Author: coolaj86