feat: default to the public corpus so the package works unconfigured

The default was a REPLACE_WITH_SHARE_TOKEN sentinel and no document in
the package supplied a working URL, so a new user installing uscogdata
had no path to a session at all -- just an actionable-looking error with
nothing actionable behind it.

The default is now the public HuggingFace mirror: CC-BY-4.0, no
credential, CDN-backed, and it keeps the origin's uplink out of the read
path. USCOGDATA_URL and options(uscogdata.url=) still override, so
Nextcloud and cog_mirror() copies are unaffected.

The sentinel guard stays for half-edited configs; the two tests covering
it set the URL explicitly, so they only needed renaming to stop calling
it 'the default'.

R/manifest.R

@@ -28,7 +28,7 @@
       "*" = "{.code Sys.setenv(USCOGDATA_URL = \"<url-or-local-path>/\")}",
       "*" = "{.code options(uscogdata.url = \"<url-or-local-path>/\")}",
       i = "For an offline smoke test, use the bundled fixture: {.code system.file(\"extdata/fixture_corpus\", package = \"uscogdata\")}.",
-      i = "For the live Civilytics corpus, request the Nextcloud share URL from the package maintainer."
+      i = "The public corpus is the default: unset USCOGDATA_URL to use it, or point it at a local copy made by {.code cog_mirror()}."
     ), class = "uscogdata_url_not_configured")
   }
   invisible(url)