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>

README.md

@@ -83,7 +83,7 @@ releaser --branch-pattern "^(?:.*/)?(?:release|hotfix)/(\d+)\.(\d+)$"
 
 ```yaml
 git:
-  tag_prefix: "v"                                    # set to "" for tags without prefix
+  tag_prefix: ""                                     # default: no prefix; set to "v" for v-prefixed tags
   branch_pattern: "^(?:.*/)?release/(\\d+)\\.(\\d+)$"  # two capture groups: major, minor
   commit_message: "chore(release): {version} [skip ci]"
   author_name: ""                                    # defaults to git config user.name