fix(sync): point sync.sh at this repo's plugin cache, not the private repo's (#402)

sync.sh was written against the layout of mvanhorn/last30days-skill-private
(`.../cache/last30days-skill-private/last30days-3/{version}`) and that path
was never updated when this public repo got its own copy. Running sync.sh
from here populated the BETA channel's cache (`/last30days-beta`) instead
of this repo's own `/last30days` cache, so devs working in this repo could
not test their changes via the public slash command without waiting for a
marketplace release.

Path now derives from this repo's own manifests:
- marketplace name `last30days-skill` (.claude-plugin/marketplace.json)
- plugin name      `last30days`       (.claude-plugin/plugin.json)

Drops the `last30days-3-nogem` target along with it - that's a private-repo
variant with no public equivalent.

Updates test_sync_cache_path_uses_skill_version to assert the new path
pattern and clarifies the COMMON_TARGETS comment so the next person editing
it understands which marketplace/plugin name segments come from where.

tests/test_version_consistency.py

@@ -24,7 +24,10 @@ class TestVersionConsistency(unittest.TestCase):
     def test_sync_cache_path_uses_skill_version(self) -> None:
         sync_text = (SKILL_ROOT / "scripts" / "sync.sh").read_text(encoding="utf-8")
         version = _skill_version()
-        self.assertIn(f'last30days-3/{version}"', sync_text)
+        self.assertIn(
+            f'last30days-skill/last30days/{version}"',
+            sync_text,
+        )
 
     def test_memory_save_dir_uses_single_env_variable(self) -> None:
         skill_text = (SKILL_ROOT / "SKILL.md").read_text(encoding="utf-8")