v0.6.2

Makes each repo's doc-graph self-navigating from graph.json.

• docGraph.url — each component's doc-graph digest now carries a url linking
  straight to that repo's full doc-graph payload (e.g.
  /repos/github.com/lockyc/mycelium/docgraph.json). A consumer reading the digest
  in graph.json follows the link instead of translating the component's id
  into the route by hand. The digest still answers "healthy / how big" inline; the
  url is the one extra hop, and only when you want to walk the full graph.
• The route is single-sourced (RepoDocGraphRoute / the RepoDocGraph*
  constants): the hub stamps the url, writes the payload, and the serve handler
  parses the same prefix/suffix — the link a consumer follows and the route the hub
  answers cannot drift.

Additive and backward-compatible: url is a new field on the existing docGraph
digest (schemaVersion 1 unchanged); relative, so it resolves against whichever hub
served the graph.

Full Changelog: v0.6.1...v0.6.2