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/shared/src/index.ts

@@ -49,6 +49,7 @@ export type {
   AdapterEnvironmentTestResult,
   AssetImage,
   Project,
+  ProjectGoalRef,
   Issue,
   IssueComment,
   IssueAttachment,