feat(config): change default tag_prefix to empty (no prefix)

Tags are now bare version numbers by default (e.g. 1.2.3).
Set tag_prefix: "v" in .releaser.yml or pass --tag-prefix v to opt in
to the v-prefixed convention.

Updated all affected tests, the .releaser.yml template comment, and
the README configuration reference.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/config/config.go

@@ -40,7 +40,7 @@ type GitLabConfig struct {
 func defaults() Config {
 	return Config{
 		Git: GitConfig{
-			TagPrefix:     "v",
+			TagPrefix:     "",
 			BranchPattern: branch.DefaultBranchPattern,
 			CommitMessage: "chore(release): {version} [skip ci]",
 		},