feat(11-01): add group table DDL and built-in group seeding to skill-registry-db

- Import LibSQLClient type for seedBuiltinGroups parameter typing
- Add DDL constants for 5 new tables: skill_groups, skill_group_members,
  skill_group_inheritance, agent_skill_groups, agent_skills
- Add BUILTIN_GROUPS constant with 5 entries (pm-essentials, engineer-core,
  frontend, backend, creative)
- Add seedBuiltinGroups() using INSERT OR IGNORE for idempotent seeding
- Extend getSkillRegistryDb() to execute all 5 new DDL statements and seed

server/src/services/skill-registry-db.ts

@@ -1,7 +1,7 @@
 import { mkdir } from "node:fs/promises";
 import { dirname } from "node:path";
 import { drizzle } from "drizzle-orm/libsql";
-import { createClient } from "@libsql/client";
+import { createClient, type Client as LibSQLClient } from "@libsql/client";
 import * as schema from "./skill-registry-schema.js";
 import { resolveSkillRegistryDbPath } from "../home-paths.js";
 
@@ -50,6 +50,85 @@ CREATE TABLE IF NOT EXISTS community_ratings (
   source TEXT
 )`;
 
+const CREATE_SKILL_GROUPS_TABLE = `
+CREATE TABLE IF NOT EXISTS skill_groups (
+  id TEXT PRIMARY KEY,
+  name TEXT NOT NULL,
+  description TEXT,
+  is_builtin INTEGER NOT NULL DEFAULT 0,
+  created_at INTEGER NOT NULL,
+  updated_at INTEGER NOT NULL
+)`;
+
+const CREATE_SKILL_GROUP_MEMBERS_TABLE = `
+CREATE TABLE IF NOT EXISTS skill_group_members (
+  group_id TEXT NOT NULL,
+  skill_id TEXT NOT NULL,
+  added_at INTEGER NOT NULL,
+  PRIMARY KEY (group_id, skill_id)
+)`;
+
+const CREATE_SKILL_GROUP_INHERITANCE_TABLE = `
+CREATE TABLE IF NOT EXISTS skill_group_inheritance (
+  child_group_id TEXT NOT NULL,
+  parent_group_id TEXT NOT NULL,
+  PRIMARY KEY (child_group_id, parent_group_id)
+)`;
+
+const CREATE_AGENT_SKILL_GROUPS_TABLE = `
+CREATE TABLE IF NOT EXISTS agent_skill_groups (
+  agent_id TEXT NOT NULL,
+  group_id TEXT NOT NULL,
+  assigned_at INTEGER NOT NULL,
+  PRIMARY KEY (agent_id, group_id)
+)`;
+
+const CREATE_AGENT_SKILLS_TABLE = `
+CREATE TABLE IF NOT EXISTS agent_skills (
+  agent_id TEXT NOT NULL,
+  skill_id TEXT NOT NULL,
+  installed_at INTEGER NOT NULL,
+  PRIMARY KEY (agent_id, skill_id)
+)`;
+
+const BUILTIN_GROUPS = [
+  {
+    id: "builtin/pm-essentials",
+    name: "PM Essentials",
+    description: "Core planning and project-management skills",
+  },
+  {
+    id: "builtin/engineer-core",
+    name: "Engineer Core",
+    description: "Foundational engineering skills",
+  },
+  {
+    id: "builtin/frontend",
+    name: "Frontend",
+    description: "UI and frontend development skills",
+  },
+  {
+    id: "builtin/backend",
+    name: "Backend",
+    description: "API, database, and infrastructure skills",
+  },
+  {
+    id: "builtin/creative",
+    name: "Creative",
+    description: "Writing, branding, and creative production",
+  },
+] as const;
+
+async function seedBuiltinGroups(client: LibSQLClient): Promise<void> {
+  const now = Date.now();
+  for (const group of BUILTIN_GROUPS) {
+    await client.execute({
+      sql: `INSERT OR IGNORE INTO skill_groups (id, name, description, is_builtin, created_at, updated_at) VALUES (?, ?, ?, 1, ?, ?)`,
+      args: [group.id, group.name, group.description, now, now],
+    });
+  }
+}
+
 export async function getSkillRegistryDb(): Promise<SkillRegistryDb> {
   if (_db !== null) return _db;
 
@@ -64,6 +143,14 @@ export async function getSkillRegistryDb(): Promise<SkillRegistryDb> {
   await client.execute(CREATE_SKILL_FILES_TABLE);
   await client.execute(CREATE_COMMUNITY_RATINGS_TABLE);
 
+  await client.execute(CREATE_SKILL_GROUPS_TABLE);
+  await client.execute(CREATE_SKILL_GROUP_MEMBERS_TABLE);
+  await client.execute(CREATE_SKILL_GROUP_INHERITANCE_TABLE);
+  await client.execute(CREATE_AGENT_SKILL_GROUPS_TABLE);
+  await client.execute(CREATE_AGENT_SKILLS_TABLE);
+
+  await seedBuiltinGroups(client);
+
   return _db;
 }