feat(schema): Task.repo_url — cross-repo override for agent worktree

When a task targets a different repo than its parent product (e.g. an
MCP-server task tracked under the main product's PBI), product.repo_url
points to the wrong place. Result observed in story 'Verify-gate'
batch (2 May): agent's gate-task work failed to push because product
was Scrum4Me but the code lives in scrum4me-mcp.

Add optional `Task.repo_url` (TEXT, nullable). scrum4me-mcp's
`resolveRepoRoot` will read this in a follow-up PR — null falls back
to product.repo_url, preserving current behaviour for the 99% case.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

prisma/migrations/20260502175000_add_task_repo_url/migration.sql

@@ -0,0 +1,4 @@
+-- AlterTable: optional repo_url override on Task. Used when an MCP-server
+-- task is tracked under a different product's PBI (cross-repo support).
+-- Falls back to product.repo_url when NULL.
+ALTER TABLE "tasks" ADD COLUMN "repo_url" TEXT;

prisma/schema.prisma

@@ -254,6 +254,11 @@ model Task {
   status              TaskStatus @default(TO_DO)
   verify_only         Boolean        @default(false)
   verify_required     VerifyRequired @default(ALIGNED_OR_PARTIAL)
+  // Override product.repo_url for branch/worktree/push purposes. Set when
+  // a task targets a different repo than its parent product (e.g. an
+  // MCP-server task tracked under the main product's PBI). Falls back to
+  // product.repo_url when null.
+  repo_url            String?
   created_at          DateTime       @default(now())
   updated_at          DateTime   @updatedAt
   claude_questions    ClaudeQuestion[]