build: add Spigot plugin foundation

src/main/java/games/dmg/spigotharvest/SpigotHarvestPlugin.java

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+package games.dmg.spigotharvest;
+
+import org.bukkit.plugin.java.JavaPlugin;
+
+/** Spigot Harvest plugin entry point. */
+public final class SpigotHarvestPlugin extends JavaPlugin {
+    @Override
+    public void onEnable() {
+        // Feature services are registered by their corresponding user stories.
+    }
+}

src/main/resources/plugin.yml

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+name: SpigotHarvest
+version: ${version}
+main: games.dmg.spigotharvest.SpigotHarvestPlugin
+api-version: "1.20"
+description: Progression-gated connected crop harvesting.
+author: dmg.games
+commands:
+  harvest:
+    description: View crop progression and manage harvest settings.
+    usage: /harvest [status [crop]|bossbar <enable|disable>]
+  harvestadmin:
+    description: Administer Spigot Harvest progression.
+    usage: /harvestadmin <subcommand>
+    permission: spigotharvest.admin
+permissions:
+  spigotharvest.admin:
+    description: Allows administration of Spigot Harvest.
+    default: op

src/test/java/games/dmg/spigotharvest/PluginMetadataTest.java

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+package games.dmg.spigotharvest;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+import java.io.InputStream;
+import java.util.Map;
+import org.junit.jupiter.api.Test;
+import org.yaml.snakeyaml.Yaml;
+
+final class PluginMetadataTest {
+    @Test
+    void declaresHarvestCommandsAndAdminPermission() {
+        InputStream stream = getClass().getClassLoader().getResourceAsStream("plugin.yml");
+        assertNotNull(stream);
+
+        Map<?, ?> plugin = new Yaml().load(stream);
+        assertEquals("SpigotHarvest", plugin.get("name"));
+        assertEquals("games.dmg.spigotharvest.SpigotHarvestPlugin", plugin.get("main"));
+        assertEquals("1.20", plugin.get("api-version"));
+
+        Map<?, ?> commands = (Map<?, ?>) plugin.get("commands");
+        assertNotNull(commands.get("harvest"));
+        assertNotNull(commands.get("harvestadmin"));
+
+        Map<?, ?> permissions = (Map<?, ?>) plugin.get("permissions");
+        assertNotNull(permissions.get("spigotharvest.admin"));
+    }
+}