feat: add multi-repo workspace support

Allow a task to have multiple workspaces, one per repository. This enables
cross-repo work (e.g., posthog + posthog-js + posthog.com) in a single
conversation.

Changes:
- DB migration: drop UNIQUE on workspaces.taskId, add label column, add index
- WorkspaceRepository: add findAllByTaskId for multi-workspace queries
- WorkspaceService: createWorkspace accepts additionalRepos, returns array
- Workspace schemas: add label field, RepoWorkspaceConfig, array outputs
- Task type: add repositories[] alongside deprecated repository field
- Sidebar: add additionalRepositories to TaskData for multi-repo badge
- useWorkspace hook: backward-compatible single-workspace access + new
  useTaskWorkspaces/useAllWorkspaces for multi-repo consumers
- Archive/suspension services: handle multi-workspace per task
- Fix downstream consumers (code editor, DraggableTab) for array workspace API

Generated-By: PostHog Code
Task-Id: 230ec8e6-6781-43b4-ae30-ea24faa410dc

Author: richardsolomou

apps/code/src/main/db/migrations/0005_multi_repo_workspaces.sql

@@ -0,0 +1,6 @@
+-- Drop the unique constraint on task_id to allow multiple workspaces per task
+DROP INDEX IF EXISTS `workspaces_taskId_unique`;--> statement-breakpoint
+-- Add a non-unique index for query performance
+CREATE INDEX `workspaces_task_id_idx` ON `workspaces` (`task_id`);--> statement-breakpoint
+-- Add label column for display name (e.g., "posthog-js")
+ALTER TABLE `workspaces` ADD `label` text;