Support PACKAGE@VERSION tag format

Summary

- Accept PACKAGE@VERSION and PACKAGE@vVERSION as singular announcement tags, alongside PACKAGE-VERSION and PACKAGE/VERSION.

Why

- @ is a common, unambiguous separator (e.g., npm uses it).
- Improves readability with hyphenated names (foo-bar@1.2.3).
- GitHub tag filters already match semver; only parsing blocks this.

Examples

- foo@1.2.3 → singular for “foo”
- bar@v0.9.0 → singular for “bar”

Proposal

- Extend axotag parsing to treat @ as a valid separator before the version (strip optional leading v, then parse semver).
- Update docs to list PACKAGE@VERSION as supported.

> **NOTE**: This issue was created with an LLM but reviewed by dionysuzx.

Start at the axotag parsing entry point and trace how PACKAGE-VERSION and PACKAGE/VERSION are recognized and how semver handles an optional leading v. Add coverage for PACKAGE@VERSION and PACKAGE@vVERSION, update the documentation to list the new format, and confirm the existing formats still work.

Written by the indexing model from the issue text.