fix(versioning): accept initial version for newly introduced plugins

[rootkiller6788]

What

version-plugins.py auto-bumps catalog plugin versions and validates builder-set bumps. When a plugins.d/<name>.yml is newly introduced (no yaml at the base ref), analyze_plugin sets base_version = head_version and builder_changed_version = True, so decide() runs validate_builder_version(base, head) with equal versions and reports:

! foo: version did not increase: base 1.0.0 -> head 1.0.0 (must be strictly greater)

There is no prior version for a brand-new plugin to increase from — the initial version is the builder's stamp, and it is already required to be a strict MAJOR.MINOR.PATCH by SemVer.parse in analyze_plugin. The current behavior makes every plugin onboarding fail (advisory in validate-plugins.yml, but fatal in sync-skills.yml's version-plugins.sh --apply --base HEAD step).

Change

• Add an is_new field to PluginAnalysis, set when the plugin yaml is absent at the base ref.
• In decide(), accept newly-introduced plugins (verdict accept) instead of applying the monotonic-increase check, which only makes sense when a base version exists.
• Existing-plugin behavior is unchanged: non-monotonic builder edits and oversized major skips still fail; structural/content changes still auto-bump y/z.
• Add .github/scripts/tests/test_version_plugins.py (10 tests) pinning both the new-plugin fix and the pre-existing decide()/build_plan()/SemVer behavior.