chore($skill): generate DB migration for session_skill table

CREATE TABLE session_skill with:
- session_id TEXT FK → session(id) CASCADE DELETE
- skill_name TEXT
- added_at INTEGER
- token_estimate INTEGER
- Composite PK (session_id, skill_name)
- Index on session_id for fast list queries

Author: praxstack

packages/opencode/migration/20260319111502_skill/migration.sql

@@ -0,0 +1,10 @@
+CREATE TABLE `session_skill` (
+	`session_id` text NOT NULL,
+	`skill_name` text NOT NULL,
+	`added_at` integer NOT NULL,
+	`token_estimate` integer,
+	CONSTRAINT `session_skill_pk` PRIMARY KEY(`session_id`, `skill_name`),
+	CONSTRAINT `fk_session_skill_session_id_session_id_fk` FOREIGN KEY (`session_id`) REFERENCES `session`(`id`) ON DELETE CASCADE
+);
+--> statement-breakpoint
+CREATE INDEX `session_skill_session_idx` ON `session_skill` (`session_id`);