feat: add plugin worker protocol, bootstrap and plugin_storage table

Worker-side ctx proxy plus the message envelope it talks over, and the
per-plugin key/value table behind ctx.storage with migrations for all
three dialects. Rows cascade with the plugin so uninstalling leaves
nothing behind.

drizzle/sqlite/0021_dashing_lester.sql

@@ -0,0 +1,10 @@
+CREATE TABLE `plugin_storage` (
+	`id` integer PRIMARY KEY AUTOINCREMENT NOT NULL,
+	`plugin_id` text NOT NULL,
+	`key` text NOT NULL,
+	`value` text NOT NULL,
+	`updated_at` text DEFAULT CURRENT_TIMESTAMP NOT NULL,
+	FOREIGN KEY (`plugin_id`) REFERENCES `plugins`(`id`) ON UPDATE no action ON DELETE cascade
+);
+--> statement-breakpoint
+CREATE UNIQUE INDEX `idx_plugin_storage_plugin_key` ON `plugin_storage` (`plugin_id`,`key`);