Add release: publish tagged releases on the mirror servers

`release [@name ...] <tag> [notes]` does the whole chain in one step —
create the annotated tag, push it to the internal server, then push it to
each selected mirror and turn it into a release object there. Target
selection reuses pushremote's @name mechanism, so the two behave alike.

Notes are generated when none are given: the tag's own annotation when it
carries more than the default, otherwise the commit subjects since the
previous tag, capped at 50 lines. `tag add v1.0 "why this exists"` now
takes a message, which is what that fallback reads; before, the
annotation was always just the tag name.

Tags ending in -rc/-alpha/-beta/-pre are marked as pre-releases on Gitea
and GitHub. Releasing the same tag twice updates the existing release;
a tag that already points at a different commit stops the command, since
moving a published tag makes one version mean different things per
server. A repository that is not on the mirror yet is reported instead of
being created as a side effect.

Binary assets are deliberately out of scope: Gitea attaches them to the
release, GitHub uses a separate upload host, and GitLab does not host
them at all but wants a link into its package registry.

The providers differ in path shape and field names -- GitLab addresses
projects by URL-encoded path, calls the notes "description" and has no
pre-release flag -- so this comes with a recording httptest stand-in that
asserts the exact requests for all three. That harness also covers
authUser, repoExists and the auth header forms, which had no test at all.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

alias.go

@@ -47,7 +47,7 @@ var builtinCmds = map[string]bool{
 	"pushremote": true, "overview": true, "archive": true, "init": true,
 	"login": true, "cd": true, "checkout": true, "clone": true, "cloneall": true,
 	"open": true, "view": true, "count": true, "tag": true, "alias": true,
-	"unalias": true, "config": true,
+	"unalias": true, "config": true, "release": true,
 }
 
 func isBuiltin(name string) bool { return builtinCmds[name] }