Rebind developer sessions instead of continuing

[zccz14]

Summary

• Remove the OpenCode session manager continueSession API so existing sessions are only advanced by the session-manager polling path.
• Rebind assigned pending Developer tasks to a newly created OpenCode session with fresh prompt context via taskRepository.updateTask.
• Update regression coverage for assigned pending rebind behavior and manager API boundaries.

Tests

• pnpm exec vitest run --config vitest.workspace.ts --project api modules/api/test/developer.test.ts modules/api/test/opencode-session-manager.test.ts
• pnpm --filter ./modules/api run test:type
• pnpm --filter ./modules/api run test:lint
• pnpm --filter ./modules/api run test:vitest
• pnpm run test:changeset