Serve repositories from the root of the URL space

Serve repositories from the root of the URL space

A repository is now /<name> and its Git transport /<name>.git, so the URL a
person opens, the URL a clone uses, and the Go module path are the same name.
The forge's own pages moved under /-/ (admin, accounts, the directory, the
feed, sign-in, assets), a prefix no repository name can start with: names are
[a-z0-9...] and cannot begin with "-". That leaves the router with two disjoint
spaces and removes the /r/ and /git/ prefixes, which existed only to keep
repositories away from the forge's own paths.

Routes: repoRoutes lose /r/; global, auth and admin routes gain /-/; assets
move to /-/static/ because a subtree pattern like /static/ cannot coexist with
one-segment-wildcard repo routes — Go's mux reports them as conflicting
neither-more-specific patterns. The catch-all still renders the styled 404.

Transport: githttp.Route parses /<name>.git/<rest> instead of a /git/ prefix,
and forgepaths.go hands such requests to the proxy before the mux, because a
wildcard fills a whole segment and cannot match "<name>.git". LFS stays on the
mux (<name>.git/info/lfs/...), and the Go module discovery probe moves to the
same pre-router so a package path under a public repository is answered even
when its first segments name a page tab.

Names: repo.ValidateName now refuses the four root paths the forge must keep —
static, robots.txt, sitemap.xml, sitemap-pages.xml. Everything else is an
ordinary repository name, including admin, u, and activity.

Docs and the packaged URLs follow the new layout; robots.txt, the sitemaps and
the instance activity feed point at the new paths. No aliases: the old /r/ and
/git/ URLs are gone.

Verified with ./vole workflow check and ./vole workflow run verify.