feat: add project_goals many-to-many join table

Create project_goals join table with composite PK (project_id, goal_id),
backfill from existing projects.goal_id, and update the project service
to read/write through the join table. Shared types now include goalIds
and goals arrays on Project. Legacy goalId column is kept in sync.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/db/src/schema/index.ts

@@ -6,6 +6,7 @@ export { agentRuntimeState } from "./agent_runtime_state.js";
 export { agentTaskSessions } from "./agent_task_sessions.js";
 export { agentWakeupRequests } from "./agent_wakeup_requests.js";
 export { projects } from "./projects.js";
+export { projectGoals } from "./project_goals.js";
 export { goals } from "./goals.js";
 export { issues } from "./issues.js";
 export { issueApprovals } from "./issue_approvals.js";