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>

main.go

@@ -147,7 +147,7 @@ func parseArgs() (int, string, bool) {
 		"clone": 2, "init": 2, "log": 2,
 		"push": 1, "pushremote": 1, "list": 1, "tag": 1, "archive": 1, "show": 1, "open": 1,
 		"pull": 1, "fetch": 1, "status": 1, "diff": 1, "overview": 1,
-		"config": 1, "count": 1, "login": 1, "cloneall": 1,
+		"config": 1, "count": 1, "login": 1, "cloneall": 1, "release": 1,
 	}
 	if c, ok := cls[a0]; ok {
 		return c, strings.Join(os.Args[1:], " "), false